Usually if I have a choice I play female in most games.
However with Fallout 3 / New Vegas I feel compelled to go as male.
Because since its a post apocalyptic world doesn't it seem like men would have a better chance of surviving?
You have probably seen in post apocalyptic movies where the women get treated like shit. Abused, used for sex and living in conditions less great then men.
Men just seem to be the dominant sex in this kind of hellish environment.
Seeing as 99% of both men and women have been wiped out, I don't see any reason why female PCs aren't reasonable or would have a lower chance of survival. Being male certainly didn't help the vast majority of the male population from getting nuked, eaten by super mutants or radscorpions, dragged off into slavery etc, and being a little stronger, on average, isn't much use when fighting off deathclaws or power-armoured soldiers that are 500+% stronger than both sexes anyway. Speaking of which, the remnants of technology lying around do a lot to even the odds. I'd rather be a female vault-dweller with a machine gun, pip-boy and power armour than a male bandit!
Besides, In RPGs, it's often assumed that the PC is an exceptional individual in terms of ability, or at least in terms of potential. It's a fantasy world, just as much as post-apocalyptic films are.