A gay man, his partner, and their adopted child move next door to you. They mow the lawn. They play basketball with their kid. They wash the car in the driveway. They are not having hedonistic orgies in the back yard, or dancing up and down the street in shiny thongs.
How does this family harm you?
Since we have separation of church and state in this country, what is your legal rationale for denying gays marriage rights?
Homosexuality exists in nature. It is demonstrably not a choice. But, if you're going to claim that homosexuality is a choice, then you will have to concoct a motive for someone in middle or high school to make a life-defining choice that will make them an outcast and second class citizen. I'm pretty creative, but I can't figure out a reason why a teenager would make a choice that makes being a teenager EVEN HARDER than it already is. So why would anyone do that?
Of course, the answer is no one would. And that bigotry and fear are the only reasons any of you oppose gay marriage.
Still haven't managed to list one way that gay marriage harms anyone. And the government not respecting your religious beliefs doesn't count, because the government is not supposed to respect people's religion. It is supposed to ignore people's religion.
Until you can find a way gay marriage harms you, you have no case. The end.