Just because it's occurring in the animal world doesn't mean it's natural, sometimes animals give birth to deformed babies does that mean deformed babies are natural ? No, the default thing is being straight not homosexual, straight is the normal and homosexual is the unnormal, the male body was not designed to be penetrated unlike the female body, that's why almost all species out there have tow sex, male and female, it's the only natural and logical thing.
The context of the word 'natural' can be broadly construed to mean that if something occurs in nature, then that something is natural.
The other context is is more narrowed. Regarding reproduction, what is 'natural' is how a species reproduce according to how it evolved. Complex species like humans cannot reproduce asexually. Less complex species are born hermaphrodites, meaning it is both male and female, then when the environmental conditions are appropriate, the animals change sex and reproduce sexually. Human beings cannot do this. So under this context, homosexuality is unnatural, regardless of how often it occurs in complex non-human species.
As a heterosexual man, I do not find other men sexually attractive, even though I may appreciate exceptional physical excellence in men, such as how facially handsome some men are, same for bodybuilders and high performance athletes. I can compliment a handsome heterosexual man about him being good looking, and I have in the past, without both of us feeling awkward about it.
The problem here is not what is 'natural' or 'unnatural'. It is how we treat fellow human beings in the humane and the legal. So what if two men or two women are in homosexual relationships ? What does that do to
YOU personally, as in what physical harm does such relationship have on
YOU ? Homosexuality may offends your religious beliefs, but so can someone who is a Christian because his/her religion offends your religious beliefs.
Bottom line is this: The degree of what offends you and how you react depends solely upon
YOU and not so much about how 'natural' or 'unnatural' that thing might be. If you kill a snake after it bit you, that is completely justified. But two men holding hands in front of you ?
No not really, pedophiles also argue that they're not mentally disordered, they claim that they're born that way and that they didn't choose it (also enhancing the chance of mental disorder), have you ever heard of the North American Man/Boy Love Association ? here's their website:
NAMBLA
they're one of the biggest pro-pedophilia organisations, they claim that the relationship between adults and children are love and a romantic relationship and they argue that there's nothing wrong with it. So yes there's a comparison.
This is about consenting adults. But even if homosexuality is a mental disorder, there are plenty of mental disorders that we live with without getting enraged about them. Those with Down Syndrome, for example. They have diminished intellectual development and are 'funny' looking. Do you support persecuting them ? Would not surprise me if you do.