From what I can see, have read, and heard from dropping in on various convos, it`s less sex itself and more the investment required to access it regularly.
Women used to at least send the message consistently, whether wife or prostitute, that access to their bodies would, one, cost him something, and two, cost him whatever investment *she* deemed appropriate. Men did not feel entitled to women's bodies as they do now. If a woman wanted $5 for it or a lifelong commitment, she set the terms without being made to feel like she had to adjust because men didn't think it was worth it. And amazingly, men accepted this. Now women are told that they are unrealistic for expecting what women have expected for generations. Give him what he wants or you'll get nothing.
I had a couple of experiences being approached after moving to DC that convinced me that legal prostitution makes more sense than whatever we have going on right now.