I'm a guy in real life and I tend to decide based on what class/race it is and what the casting will look like. For example I prefer orc rogue to say undead because, as combat, my orc swings look like I'm hitting you hard but then I think an orc male's casting just isn't what I like compared to say a female undead.
As a result of me preferring caster classes then melee I find myself playing female characters over male ones with few exceptions (male tauren and orcs). Plus I figure if I'm going to be staring at the arse of my character all night then I might as well make it a female one rather then a males. This might come across as "sexist" or whatever but its not meant to be its just the choice I make based on how the monkeys hard wired my brain. :S