We're driven towards intimacy, it seems. And if we can't get 'vanilla' intimacy, our desires get more and more bizarre. Another commenter in this thread quotes TFA where the author says something like 'but chatting sexually lost its meaning when I realized everyone else was doing the same thing with her'.

It is fascinating to see axioms of human relations come bubbling up in the most interesting places; the desire for intimacy, the desire for monogamy.