I don't but by definition doing so is definitely a perverted way of orienting pleasure meant for reproduction purpose.
No, that's religious bullshit, just moral judgment. Fortunately, freewill allows human being to have all kind of sex only for pleasure without caring about reproduction.
When you attribute perversion to someone or someone, you're doing a moral judgment, it cannot be inferred by any science field. Moral judgments comes from culture and religion.
That requires you to first make the moral judgement that perverted is bad. Which someone could argue was not the case. Perverting the justice of an unjust law for example:P
That is like asking, "Why are you assuming a digestive purpose on eating?" Sex is for the purposes of reproduction - that's just fact. Keep your strawmans to yourself.
One thing human development has achieve is to split sex from reproduction. Humans will have sex for fun, it's human nature. You cannot go against it, religions have failed.
That doesn't change what nature's intent for your organs is. You are free to engage in crimes against nature, but don't try to convince people its "normal".
Natural selection IS nature, you can't go against it unless you like eugenesia and reducing genetic pool. Again: crime is a moral judgement, not science.
"If homosexuality is discarded by natural selection, why it still exist? " I'm sure non-stop promotion in the media helps. Literally every TV show has gay character by the 2nd season.
Illnesses have been always present in nature. Contracting them is certainly not an advantage since your genes are being removed from the genpool (inability to procreate).