My experience is that guys are much less likely to talk about sex when women are around, and vice versa.

Before you ask, I was in an office surrounded by women for a year while in the Navy, and I became "one of the girls." Hence my insight into girltalk.

I've also noticed that a woman who is around enough to be "one of the guys" gets treated similarly, except men are MUCH more likely to get embarrassed when they realize a woman is present.