i think the fundamental differences in the way most men and most women view sex is you answer. I'm not wanting to make sweeping generalisAtions here, but in my opinion for men sex is a means to an end (orgasm) whereas i think sex is more emotional for women (in general) therefore they "give" more of themselves when sleeping with someone, Meanng it's more precious to them = payment perhaps.
I think you are more likely to find men who have more of a need for sex than women, as (again, my opinion) men are more driven by their sexual urges