The difference is that a criminal harms others by their criminal act while two people of the same sex doesn't.
Yes, but all behavior has a genetic element, so I could justify anything I do by saying I was born that way. Again, I'm not criticizing being gay, I just think you can come up with better justifications than that.
I'm genetically predisposed to want to have sex with lots of women, but I don't because my wife doesn't understand. And I'm sure she won't accept the I was born this way argument.