Even if the stigma were removed, I think the years of human evolution which encourages men and women to pair-wise mate for life would make it hard on an emotional level for more people to provide sex as a service. Outside of our biological needs, the health risks would be difficult to manage as well. Perhaps if technology caught up and allowed for an immediate STD screening that would mitigate those risks significantly, and/or if every possible STD was treatable, then sure. There would still be the physical/sexual assault risks to contend with.