My reasoning for it would be simple: Because i can.
Its really just that. I can RP any type of Character, including the gender convincingly enough. Which would mean that if any of ya ever met me in and Game which included RP, i.e. MMOs, Forums etc, you wouldnt go "Well thats a Dude/Chick" based on how i act.
I make a Character based on a initial Idea, usually the female Option lends itself better to it, for example my last playthrough of FO:NV was as a female unarmed/melee Specialist. I found it somewhat hilarious that the "weak gender" could kill most people with 2-3 Hits (Ballistic Fist ftw). Mass Effect the reason is basicly Jennifer Hale. Better delivery than Meer, so thats all there is to it.
I dont think though that picking a Gender in a Game somehow implies you are that Gender in RL or wish to be. Hell if asked about which RL gender i am i'd answer "neither" just because it does not matter in that context. Do you play a MMO so you can meet with Guys/Girls? in RL? I dont, so it doesnt matter whether i am my chosen "virtual gender" or not. By any Definition, the Player on the Internet has no Gender aside from what he wants to portray.
So i play both depending on various Reasons, mostly based on how i feel like at the time though