You may think being gay is "unnatural", but for those who are gay it's the most natural thing in the world, it's trying to be something they are not that is unnatural.
I'm talking about nature as in the purpose of biological functions and substances (i.e. sperm, eggs, male and female reproductive systems, etc.). If you want to talk about feelings and desires that seem natural to people, you could also take a survey of child molesters and I'm pretty sure a large number of them would say that being sexually attracted to children is the most natural thing to them in the world as well.
I'm not saying that homosexuals and child molesters are the same thing. I'm saying this only to show that your logic is flawed. To say that non-heterosexual sex is unnatural has nothing to do with desires that seem to come natural to people, but rather, it has to do with biological functions in nature.