Percy Q. Shunn basically beat me to it, I was gonna say that homosexual marriage is a threat to hetero marriage like vegetarians are a threat to meat-eaters.
I have asked many fundies/homophobes/anti-homosexual marriage people to give me specific, concrete, examples of how allowing gay people to marry hurts their marriage, or affects their lives in any way. The only thing I've ever gotten that remotely resembles an answer is someone saying that they don't want their kids to be exposed to "being gay is OK", because they believe that homosexuality is a sin. Still no threat to their marriage, the only effect to them personally is having to decide whether they want their kids to be accepting or not, and either indoctrinating or not.
Even if you believe with every fiber of your being that being gay is a sin, you cannot shield your kids forever from all the things that truly WOULD be sins (and please know that I don't believe in the concept of sin), like murder, bullying, rape, theft, etc. And let's not forget the dangers you open yourself up to by even leaving your house everyday... getting hit by a car, getting mugged, catching the flu. Or by NOT leaving your house everyday... slipping in the shower and hitting your head, eating something that causes food poisoning, having your residence catch fire...
These ignorant fools can try all they want to shield themselves and their loved ones from the "gay agenda", it's just sad that they can't see there are much greater dangers in the world than two people they don't know and will never know having proper marriage benefits.
Wow. That was a rant I wasn't expecting from myself this morning.
TL;DR version: ignorant homophobes are pathetic.