I would say that there is almost always an "exchange" that occurs in regards to sex, especially in relationships. When a girl is putting out on the regular, they want some sort of collateral, which usually equates to commitment, money, or emotional involvement.
It's not that they don't enjoy sex on a raw physical level, it's just that sex is almost NEVER a stand-alone activity for them. It has to be accompanied by some form emotional value or security assurance. This is the way they are wired. They know that if there is one thing men cannot go without, it is sex; so it follows that conditions can be placed on a steady flow of pvssy.
I think a woman's need for security and valuation comes before sex. I know not everyone agrees with this, and I will add the caveat that when a woman wants to get laid she will certainly get it, but in the scheme of a female's life there is a hierarchy of needs: Attention (valuation), security (provision), validation, and support.
Men, on the other hand, need an adventure to live (a mission), a battle to fight (a project, a struggle, or something to conquer), and a woman to care for him physically. It's not that we don't desire security in some forms or emotional closeness with a woman, but our life MISSION will (or should) mostly have priority with respect to relational needs. This is part of what makes a man, a Man. Women are generally better keepers of relationships and family, while men are the builders, the providers, and the explorers. That is NOT to absolve men from pulling their weight in a relationship, but people don't want to recognize this truth anymore.