Young women as well as young men can and will behave foolishly....but the downside for the gals is steep.
Even if you take pregnancy out of the mix, the downside for women is much greater. One of the biggest dangers IMO is that a woman in her early 20's has a huge relative advantage over a guy in his early 20's because older guys are hitting on them too. But after 25, a woman's bargaining strength realative to men her age starts going downhill very quickly as those older guys are looking for the new flavor of the week. By 28, if she hasn't recognized this fact and married a good man or nurtured very good career for herself, then she is in serious trouble.
The problem is that people tend to stick with a "good" thing. A guy can afford to party well into his 30's, but a woman really can't. That is the problem with shows like "Sex in the City"; girls see this stuff and think they can have it all, and then at age 37 marry the guy of their dreams.
Eventually, the technology will be out there to keep women as young and fertile as a 20 y.o. well into their 40's. This will be a net plus for everyone, but until then, mothers need to teach their daughters to always take the long view.