Marriage is not a right.
It is a privilege conferred by society in return for the benefits heterosexual marriage brings to society. Marriage is not a private matter but a public institution. Gay marriage is not about happiness or benefiting society but about grabbing government goodies.
I have no desire to deny gay couples happiness, but why do they need marriage to be happy? Do whatever you want -- it's your own private business. But gay marriage is about public recognition, public status, and public benefits. Unfortunately, public responsibility is not part of it.