Here's a thought: if you are against gay marriage, don't marry a gay person.
Have you seen that horrific video about the gay storm coming? (Stephen Colbert nicely mocked it on his show.) It deserves to be mocked. If these so-called "Christians" would stop using the bible to deny consenting adults their equal rights and focus on being actual, tolerant, compassionate Christians, they might brush the fog from their eyes (storm pun!) and see that the real threat to marriage - gay or straight - is domestic violence and irresponsible adults who raise irresponsible children.
Do you know why some believe that gay marriage "threatens" straight marriage? Because once gay marriage is legalized, some closeted people will realize their marriage is actually a sham. They'll think, oh, no! All this time, and I could have just married a man/woman!
Here's a stupid little parody I found: