There can be all different kinds of factors in play, and yes masculinity is often a driving force to some degree for decisions made, but what I know for certain is that we all have our needs, even women.
No man, short of church-camp guy, is going to say, "I'm really, really horny, but I want to wait on sex until I'm in a good, solid, loving relationship". Don't believe women are any different. Many may pretend to be like that, but if most women were truly honest, they would admit their minds can often be as dirty and in the sexual gutter as our own.
At some point, single women grow tired of going on 100 first and second dates that ignite no passion and go nowhere. Many times they're sexual needs aren't being met with the whole 3-5 dates of "get to know you before sex" stuff, especially if the guy turns out to be an atrocity 3 dates in and she has to start over with some other dude. Eventually, they just say "fvck it", go on a date or two, get drunk and get themselves some nasty. Then they have standards again, until the horny comes back.
There always comes a point in the life of every sexually active adult where you just can't hold out and wait for that "great" person anymore, because it takes too damn long to find them. The same is true of people coming out of dead relationships. Rebound sex. Perfect example. Not looking for the LTR. Just gotta get some for the itch.
We all have our needs, and we're all going to appease them when the time it takes to hold up on high standards is grinding on us.