Why would anyone even want to sleep around?

Ayn Rand used Francisco D'Anconia to describe her view of sexuality in Atlas Shrugged, but while her explanation was easy enough to understand, there were some things she left out. Namely: why would someone, anyone, sleep around? I've met, and read articles by, women who describe their experiences in the "hookup" culture, and across the board they agree that most of the men they slept with were poor lovers who cared little for them once the act was finished. I know men like this in real life who seem surprised at how unfulfilling their sex lives (admittedly much more active than mine) really are. So I have to ask: why would someone choose to have sex with someone when they know, or at least have no good reason to not believe, that the person has no actual interest in them personally?