Those who oppose allowing gay marriage often couch their arguments in terms of "defending marriage". In fact, one of the key pieces of legislation on this issue is entitled the Defense of Marriage Act (enacted in 1996 and signed into law by President Clinton.)
Excuse me?? How does allowing a gay couple to marry, and have all the civil benefits of a marriage, in any way damage traditional marriage? I am in a happy traditional marriage, and I see no threat at all to my own marriage when gay couples marry. A gay couple marrying somewhere doesn't change my own relationship to my own spouse, nor do we lose any civil or legal rights.
No, gay marriage is no threat whatsoever to traditional marriages.
The only threat is to some people's own religious ideology. I believe in religious freedom (so long as it doesn't include honor killings, oppression, or human sacrifice), so I respect their right to hold and follow their own beliefs in their own lives, but I recognize no right for them to impose their beliefs on the rest of us and insist that we live our own lives according to their own peculiar religious ideologies.