Guys sit on toilets sometimes, but girls don't use it when the lid is up, as far as I'm aware.

shlup's point is best; I don't want to get my hand next to yucky boypee

We had a big row in band over this three or four years ago.

Apparently some chick had left a pad or something in the floor? (I don't remember exactly) and the guys would never put the seat up. We separated into genders and had a talking to by senior members and then our dumb sexist band directors decreed that the band toilet must be kept clear of period supplies and that as men, they didn't feel they needed to put the seat down, so it was very demeaning for the women!

Also, men are supposed to be courteous.

I'm going to practice peeing standingup and just spite you guys anyways that leave the seat up. Or maybe I won['t practice and spite you anyways, and then you'll have to clean it up!