Also an English speaker, and nerd about this sort of semantics.
Just to keep the blood pressure low here: I want to be clear there are two sides to this issue. I'm not the sort of "enlightened centrist" who says that lightly: more often than not, there are not two sides to an issue, but this is one of the rare cases where there are two sides to this.
I'm going to explain this as briefly as I can, but it's complicated, so it's not actually going to be brief.
First, let's understand that communication is a two-person (or two-group) activity involving both communicator and audience. Effective communication of complex and charged topics requires that the communicator understand their audience and use language that their audience can understand without being distracted. Likewise, effective communication of complex and charged topics requires that the audience make a good-faith effort to understand the communicator by taking into account the communicator's background/motivations, and try to suss out the communicator's intended meaning, possibly by asking clarifying questions.
One of the important observations in David Foster Wallace's Tense Present[1] is that language doesn't just communicate its direct meaning, it also communicates membership in a group. As Wallace says:
> [M]ost of us are fluent in more than one major English dialect and in a large number of subdialects and are probably at least passable in countless others. Which dialect you choose to use depends, of course, on whom you're addressing. [... T]he dialect you use depends mostly on what sort of Group your listener is part of and whether you wish to present yourself as a fellow member of that Group. [... U]sage-as-inclusion is about much more than class. Here's another thought experiment: A bunch of U.S. teenagers in clothes that look far too large for them are sitting together in the local mall's Food Court, and a 53-year-old man with a combover and clothes that fit comes over to them and says that he was scoping them and thinks they're totally rad and/or phat and is it cool if he just kicks it and does the hang here with them. The kids' reaction is going to be either scorn or embarrassment for the guy — most likely a mix of both. Q: Why? Or imagine that two hard-core urban black guys are standing there talking and I, who am resoundingly and in all ways white, come up and greet them with "Yo" and call them "Brothers" and ask "s'up, s'goin on," pronouncing on with that NYCish oo-o diphthong that Young Urban Black English deploys for a standard o. Either these guys are going to be offended or they are going to think I am simply out of my mind. No other reaction is remotely foreseeable. Q: Why?
> Why: A dialect of English is learned and used either because it's your native vernacular or because it's the dialect of a Group by which you wish (with some degree of plausibility) to be accepted.
I think what a lot of conversations about the word "females" versus "women" or "girls" get wrong is making universal statements about English as if it's a single coherent language. There are many dialects, such as biology English or medical English, where using "female" is absolutely appropriate and using "woman" or "girl" would be unequivocally wrong. Likewise, there are dialects, such as gender theory English, where "female" isn't dehumanizing if used properly: using "females" refers to people with XX chromosomes, while "women" refers to people who identify with a feminine gender (although usually one would say "female assigned at birth" or "FAB" rather than saying "females" by itself).
Audience is everything here. Using gender theory English to a biology audience drastically changes the meaning, placing you as an outsider to the group, and if your intention was to speak biology English, you'd have failed to communicate in that dialect. In fact, the above paragraph, which I, a cisgender man, wrote, is fine only because the audience is a mostly-male, slightly-socially-inept audience of Hacker News denizens. If my audience were a group of American women, that paragraph would be horribly offensive because it's someone who hasn't directly experienced the effects of this sort of language, explaining it to people who have directly experienced the effects of this sort of language. In other words, mansplaining.
The problem with saying "[I]t's normal thing to see young females taking a dog out for a walk[.]" is twofold:
1. It is perceived by some as dehumanizing women. Note that I didn't say "it's dehumanizing women", because that a) assumes the word has some sort of inherent meaning (it doesn't) and b) assumes a lot of intention coming from the communicator. But actually, whether it's dehumanizing isn't as important as the fact that it's perceived as such. Remember that communication is a two-person activity. Yes, that means that it's up to the audience to not assume the worst, and to assume you aren't intentionally dehumanizing women. But it also means that as the communicator, if you find out that your audience perceives the wording as dehumanizing, it's now up to you to change your language to communicate your actual intent more effectively. If you as a communicator find out that your audience perceives your wording as dehumanizing to women, and then insist on staying with your wording, that really starts to communicate that you don't care about communicating effectively, about your audience's perceptions, or about the fact that you might be misunderstood to be dehumanizing women. In the worst case, it communicates that you do, in fact, intend to dehumanize women, although I'm not willing to go that far in your case: I think it's much more likely that you're just feeling attacked and being defensive.
2. As noted before, language communicates membership in a group. In this case, we're clearly not demonstrating membership in medical, biology, or gender theory communities, and the remaining groups which use "female" aren't groups you probably want to associate yourself with. Unfortunately, the other groups which say "females" are generally groups like incels, neckbeards, self-described misogynists, self-described alpha/sigma males, etc. Likely, this is the underlying reason why "females" is viewed as dehumanizing women: these groups really do intend to dehumanize women, and by associating yourself with them, you're implying that you agree with some of their ideas. Again, I assume it isn't your intent to associate yourself with these groups, but effective communication of your identity would avoid associating yourself with these groups. It's up to you, as the communicator, to understand your audience and how they will perceive your use of words, and to choose the words that your audience will perceive as your intended meaning.
Just to highlight the complexity of this: there are also contextual problems with referring to adult women as "girls" as this might be perceived as infantilizing. But, in other contexts, referring to adult women as "women" is denigrating to their youthfulness/attractiveness (i.e. some women would often take offense to being called "that woman" rather than "that girl"). Louis C.K. was adept enough to riff on this phenomenon with his "Nobody wants to see 'women gone wild'" joke, but we all know what happened with Louis C.K. later... But this is partly why Global Citizen has a "Girls and Women" issue rather than a "Girls" or "Women" category (and certainly not a "Females" category)[2]. Either "Girls" or "Women" would not be inclusive of the entire group they're trying to help in the context of their audience's dialect.
Just to keep the blood pressure low here: I want to be clear there are two sides to this issue. I'm not the sort of "enlightened centrist" who says that lightly: more often than not, there are not two sides to an issue, but this is one of the rare cases where there are two sides to this.
I'm going to explain this as briefly as I can, but it's complicated, so it's not actually going to be brief.
First, let's understand that communication is a two-person (or two-group) activity involving both communicator and audience. Effective communication of complex and charged topics requires that the communicator understand their audience and use language that their audience can understand without being distracted. Likewise, effective communication of complex and charged topics requires that the audience make a good-faith effort to understand the communicator by taking into account the communicator's background/motivations, and try to suss out the communicator's intended meaning, possibly by asking clarifying questions.
One of the important observations in David Foster Wallace's Tense Present[1] is that language doesn't just communicate its direct meaning, it also communicates membership in a group. As Wallace says:
> [M]ost of us are fluent in more than one major English dialect and in a large number of subdialects and are probably at least passable in countless others. Which dialect you choose to use depends, of course, on whom you're addressing. [... T]he dialect you use depends mostly on what sort of Group your listener is part of and whether you wish to present yourself as a fellow member of that Group. [... U]sage-as-inclusion is about much more than class. Here's another thought experiment: A bunch of U.S. teenagers in clothes that look far too large for them are sitting together in the local mall's Food Court, and a 53-year-old man with a combover and clothes that fit comes over to them and says that he was scoping them and thinks they're totally rad and/or phat and is it cool if he just kicks it and does the hang here with them. The kids' reaction is going to be either scorn or embarrassment for the guy — most likely a mix of both. Q: Why? Or imagine that two hard-core urban black guys are standing there talking and I, who am resoundingly and in all ways white, come up and greet them with "Yo" and call them "Brothers" and ask "s'up, s'goin on," pronouncing on with that NYCish oo-o diphthong that Young Urban Black English deploys for a standard o. Either these guys are going to be offended or they are going to think I am simply out of my mind. No other reaction is remotely foreseeable. Q: Why?
> Why: A dialect of English is learned and used either because it's your native vernacular or because it's the dialect of a Group by which you wish (with some degree of plausibility) to be accepted.
I think what a lot of conversations about the word "females" versus "women" or "girls" get wrong is making universal statements about English as if it's a single coherent language. There are many dialects, such as biology English or medical English, where using "female" is absolutely appropriate and using "woman" or "girl" would be unequivocally wrong. Likewise, there are dialects, such as gender theory English, where "female" isn't dehumanizing if used properly: using "females" refers to people with XX chromosomes, while "women" refers to people who identify with a feminine gender (although usually one would say "female assigned at birth" or "FAB" rather than saying "females" by itself).
Audience is everything here. Using gender theory English to a biology audience drastically changes the meaning, placing you as an outsider to the group, and if your intention was to speak biology English, you'd have failed to communicate in that dialect. In fact, the above paragraph, which I, a cisgender man, wrote, is fine only because the audience is a mostly-male, slightly-socially-inept audience of Hacker News denizens. If my audience were a group of American women, that paragraph would be horribly offensive because it's someone who hasn't directly experienced the effects of this sort of language, explaining it to people who have directly experienced the effects of this sort of language. In other words, mansplaining.
The problem with saying "[I]t's normal thing to see young females taking a dog out for a walk[.]" is twofold:
1. It is perceived by some as dehumanizing women. Note that I didn't say "it's dehumanizing women", because that a) assumes the word has some sort of inherent meaning (it doesn't) and b) assumes a lot of intention coming from the communicator. But actually, whether it's dehumanizing isn't as important as the fact that it's perceived as such. Remember that communication is a two-person activity. Yes, that means that it's up to the audience to not assume the worst, and to assume you aren't intentionally dehumanizing women. But it also means that as the communicator, if you find out that your audience perceives the wording as dehumanizing, it's now up to you to change your language to communicate your actual intent more effectively. If you as a communicator find out that your audience perceives your wording as dehumanizing to women, and then insist on staying with your wording, that really starts to communicate that you don't care about communicating effectively, about your audience's perceptions, or about the fact that you might be misunderstood to be dehumanizing women. In the worst case, it communicates that you do, in fact, intend to dehumanize women, although I'm not willing to go that far in your case: I think it's much more likely that you're just feeling attacked and being defensive.
2. As noted before, language communicates membership in a group. In this case, we're clearly not demonstrating membership in medical, biology, or gender theory communities, and the remaining groups which use "female" aren't groups you probably want to associate yourself with. Unfortunately, the other groups which say "females" are generally groups like incels, neckbeards, self-described misogynists, self-described alpha/sigma males, etc. Likely, this is the underlying reason why "females" is viewed as dehumanizing women: these groups really do intend to dehumanize women, and by associating yourself with them, you're implying that you agree with some of their ideas. Again, I assume it isn't your intent to associate yourself with these groups, but effective communication of your identity would avoid associating yourself with these groups. It's up to you, as the communicator, to understand your audience and how they will perceive your use of words, and to choose the words that your audience will perceive as your intended meaning.
Just to highlight the complexity of this: there are also contextual problems with referring to adult women as "girls" as this might be perceived as infantilizing. But, in other contexts, referring to adult women as "women" is denigrating to their youthfulness/attractiveness (i.e. some women would often take offense to being called "that woman" rather than "that girl"). Louis C.K. was adept enough to riff on this phenomenon with his "Nobody wants to see 'women gone wild'" joke, but we all know what happened with Louis C.K. later... But this is partly why Global Citizen has a "Girls and Women" issue rather than a "Girls" or "Women" category (and certainly not a "Females" category)[2]. Either "Girls" or "Women" would not be inclusive of the entire group they're trying to help in the context of their audience's dialect.
[1] https://genius.com/David-foster-wallace-tense-present-democr...
[2] https://www.globalcitizen.org/en/issue/women-girls/