I play a fairly even number of menfolk and women in both video games and tabletop. I have been openly told me my I cannot pull off a girl in RP. So, my girls are mannish, but I don't hide my gender behind my avatar.
Why choose the opposite sex at all? Not sure. Can't put my finger on it exactly.
Outfit choices are often the case. Dudes usually have 800 lbs of armor, shoulderpads bigger than their heads, codpieces that look like someone punched screws from the inside out. You usually don't get that sort of outrageous Mad Max thing going on with girls. Sure, there are often gross misrepresentations of body armor (exposed midriff, decolletage, rear) but for the most part you can see the person as opposed to a veritable cyborg.
What's the point of a customized burly tattoo if it's going to be covered in wrap-around platemail?