Dudes would have to sleep with each other if women didn't exist. I'm certain that this would cause a lot of wars. I just couldn't imagine what all the homophobes would do, if there was no women!
If women didn't exist, the toilet seat would never have been invented. Yes men sit, but we would probably see a toilet seat as too much of an obstacle, that would outweigh the uneasiness of not having a comfy seat.
If women didn't exist, the word, "manners", probably wouldn't either.
I could go on and on, but women simply make the world a better place. Women make men care about their appearance. Women make men have emotions, and keep us from going completely primitive. Women give us something to protect, honor, and strive for. They also challenge us, and that's a huge thing.
Women complete us. Well, they don't complete the gay men, but they do give them something to model themselves after!