You talk about nature. Do you know that in nature it's common for the females of many species to "offer" sex in exchange for material benefits? (like food) Even in species close to us, like apes.

You may go all ideological and in denial, but the fact is, thins kind of exchange is quite natural, and it always happened: think one million years ago, a man just catched a juicy prey, and a woman is hungry... what do you think she did to have some?

(BTW, I never used such services, so I'm not making excuses for myself)