Who gives a stuff. That's, like, their opinion man.
In all seriousness, as long as these people don't run for congress I couldn't care less. Here are some handy tips when arguing against these people:
Marriage is not related strictly to any religion, and has existed far before Christianity, or Islam.
As being gay is something you can't decide, denying them marriage to their loved ones is bigoted, and they should not try to control who gets marriage licenses because they forget different people than you.
If your reasons against gay marriage are religious, remember you can't control other people's rights in the name of religious freedom, nor will your religious reasons hold up alone in general US law.