"Gay marriage is immoral and sinful."
Based on?
Oh, that's right. A book written in the Bronze Age that also says snakes and donkeys can speak, a giant flood wiped out every living thing on the planet but an old drunk and his family along with all the animals they could cram onto a boat that couldn't possibly have been structurally stable at the end of which your god, apparently, magicked away all of the evidence, among other absurdities.
I don't think I'd place too much credence with such a book if I were you.
"As for not letting someone be happy, well a murderer would be happy killing people, it doesn't mean we should let them just so that they're "happy"."
That's because they would be making someone else, namely their victim but also the victim's family and friends, very unhappy.
By the way, that's a rather poor analogy to begin with. Perhaps you should rethink it. I believe the current fundy trope is to compare homosexuals to people who wish to marry their gerbil or some such.
"If gay was natural, then they would be able to have sex, but they can't, so gay isn't natural."
Uh...
