Quiksilver said:
Girls can only get pregnant for a few days each month, in a certain part of their menstrual cycle.. Chances are she was smart enough to know when it was 'safe' to fvck without gear on.
The thing is girls don't even know when they're fertile. It's called hidden estrus and a key point as to why humans have the sexual behavior they have.
Anyway, I think a lot of it has to do with getting caught up in the moment. Not only does asking about using a condom interrupt the flow of things, but I would imagine it's the last thing on anyone's mind in the heat of the moment.
Then again, most people have the "it won't happen to me" mentality when it comes to getting pregnant or getting an STD.
So there are many factors. Some girls want a baby (whether or not they admit it), some forget, and some just don't care.
Always wrap it up though. Unless you're in a LTR where you know your partner is clean it's better to be safe than sorry.