I live in Finland, where we have the civil union and there are talks about gender-neutral marriage law, and gays getting married hasn't destroyed the society or anything.
As for myself, it's not like marriage is at the present topical for me personally, but I'd like to marry at some point in my life, and it's good to know I can marry a person of same sex if I choose to. Marriage, for me, holds no religious meaning, but the commitment and legal perks of marriage do.
And well, I know people who have gotten in a civil union, and of course I'm happy for them.
Also, think of the children. There are, and will be children living with gay parents, and if they are married, it would be better for the children, since they would legally be a family, and if for example the relationship ends, the both real parents of the child will have rights, and the child will have rights to see his/her divorced parents, and other legal benefits.