This is not an argument made out of bigotry, or ignorance of what it entails for gay people. I understand that gay people want marriage very much, and I think that that is an important value. But society is made of a lot of large groups of people who want things very much, indeed as much as gay people want to get married. If--and it is a big if--allowing gays to marry would undercut the institution of marriage, then I would be against it no matter how unhappy this made gay people.
It's not your decision. Do you get that? You don't get to decide who deserves to marry and who doesn't. Who has civil rights and who doesn't. It's not your life, it's not your choice. If you are so concerned about the perpetuation of marriage, get married.