Lots of reasons, but a main one is not enough self-esteem to even consider why not. Second reason is power/control. (An illusion) And the core of that desire is anxiety. Lay over both of the above, with the fact that some females are into sex just as much as guys are. Add the aspect of possible love for the other person, which can obliterate all logic. Now add alcohol which makes a person forget all about why they weren't going to do something.