I've had this problem in doing RP's that deal with historical settings or even modern-day settings. I had one RP where I cut off the number of female characters for the RP out of considerations of realism. It doesn't come up often, but if I am doing an RP that involves a squad of modern day PMC types, which is a very male-dominated environment, I try to play true to the spirit of that, even if it means having to awkwardly explain that the characters are gender-restricted. In one specific RP, I capped off the roster at one female character and played the discomfort involved with that because it reflected a similar situation that prevails in reality. Similarly, in historical RP's, I tend to enforce the gender restriction. None of this actually reflects my personal stance, but I like to play to a sense of realism and the reality is that there is gender bias out there.

On the other hand, I'm cool with female characters pretending to be men to join the fighting as that's happened. I try to bend things to accommodate players.

I'm not sure that's a popular stance, but it's my comfort level -- if I'm doing an RP about WWII Marines in Guadalcanal, the reality is that there won't be women there. On the other hand, I try to do RP's that can accommodate female characters, like the French Resistance in WWII or the Haganah in the Arab-Israeli wars because these are groups that had women fighting on their behalf. In sci-fi environments it's easier, of course. I haven't really had this problem come up lately, but I've dealt with it by taking what I think of as an unpopular stance. On the other hand, if the right player joins up, I'll do my best to loophole it. ;)