I like boys, boys are cute
Female avatars are also cute
I think part of the reason is that female models just have a lot more stuff available to them. I can scroll through the top 100 booths or so and find mostly girl avatars, with tons of clothing options. If you want a male avatar your options are a little more limited, and chances are that if someone is using a male avatar, it's a deliberate choice to make a male avatar, they didn't just pick a random model that looks good.