This question reminds me of what I'd say if a girl asked me why men always leave the toilet seat up; "Uhm, that's how we use it!" If ya want us to leave it down, oook, but you'd not like the results I'm sure! ;)
Seriously though, in my household it's not a problem for two reasons. One, no women! Eventually I'd like it to happen but currently it's a no go. Secondly, I always put the seat and the lid down as I have cat's and don't want them having their own little swimming pools. :P
But as mentioned earlier, it's just as easy for a woman to put it down as it is for a man. If a woman is going to get all high and mighty (read bitchy I guess) about the toilet seat, then I'd come back with "Why don't you leave it up?" But, honestly I'd vote to just put both the seat and lid down.
I honestly think it's just really one of those questions that is an invitation to a fight like "Do these pants make me look fat?" type of thing.