Why do Christians want to ban gay marriage?

[tiggervandamme3000]

I don't believe in gay rights nor do I believe homosexuality is valid lifestyle. However, I don't think beating the snot out of people is a smart thing either and I'm not trying to ban ultimate fighting. Besides don't Christians believe you should come to God on your own not through civil laws? What if lusting after a woman was banned? Would I get a ticket for checking out a beautiful woman?