That Men, as a whole, are not very attractive when naked.
Sure, they might be pretty good from the waist up, but below the belt is certainly not a basket of puppies. There's just very little redeeming in that area, from the hairy legs to the hairy other parts. Even when trimmed, the best it looks like is a weird cat toy. And don't even get me started on the fun of playing. You can make things talk, make them puppets, even make it all dance.
Guys, are you uncomfortable yet?
Girls on the other hand, are much more interesting to see naked. They have differences in their bodies, more stuff to have differences in. They cover up more in public, so you've got more to leave to the imagination. Plus, girls have curves that can peak at different places, or be absent. Guys having curves... there's only one way to curve- out. A good drum, but nothing I'd like to see silhouetted.