A man is shot in New York. Because he was gay.
While there is social inequality for such people it sends a dangerous message. It says that people who aren't straight are not deserving of full respect, that society needs to exclude them. (Exclude us. I'm gay, I'm one of the excluded).
Gay marriage is essential because it's not gay marriage, it's marriage equality. Marriage should be for all, no matter what their sexual orientation. Anything else is social poison that can end up concentrated to such toxicity that it can lead to violence. Children should not grow up being taught that gay people are different in any way other than sexual orientation. This corrupts the minds of straight children and creates fear and self-loathing in those children who are not straight. Gay marriage indicates to children that their dreams of a relationship that is fully accepted by society can come true no matter who they love.
Anything but full social equality spreads fear and fuels hatred. The oppression, the bullying, the violence and the flow of blood from those cut down because of who they love has to stop.