Homosexuality is not a practice, it is a biological condition determined but an animal's sexual and romantic attractions to the same sex. You practice playing a sport or drawing or speaking a new language. You cannot practice homosexuality, it comes naturally. That being said, it's no more wrong than heterosexuality. It's simply a variation of sexuality in animals.
Humans are animals as well and homosexuality is present in multiple animals so its not surprising that it occurs in humans. I think this is a brilliant question, as it shows that you have to take a logical stand on homosexuality. Well done to MasturDbtor. Hopefully this question will make people think more. Or it will make them even more vitriolic.
I just submitted this question wondering for those people who are against humans practicing homosexuality are you also against animals practicing homosexuality? I'm for it either way (except for human on animal because there's no consent). I want to see if anyone who is anti-gay for humans is also opposed to it when animals are gay.
Those who are really homophobic will always argue at that point that humans are not animals and probably that there are not even homosexual animals as they are just too confused (because imprisoned) or unintelligent to know the difference between a male and a female and that god created them this limited.
Anyway as most animals are not even mammals it's not even possible to rate them as "homosexual". There is no such thing as "rape" for animals (as we cannot depict the level of consent) and other concepts that are VERY REAL for Humans but don't mean anything if you just leave the spectrum of mammals.
I am not saying that there are no tigers that shown tendencies comparable to those of homosexual humans but it's a wrong approach, because animals are unable to intelligently be aware of these tendencies and act due to this awareness.
Anyway - I have difficulties answering this because animals can not even choose to NOT practice homosexuality and all species do to a certain degree include same-sex-sex-acts.
I am Pro-Gay but I am pretty sure this question will not bring you the advantage in a Pro-Homosexuality-Argument that you search for. It already bases on the concept of being "gay by birth" that you are trying to communicate. So you would need them to understand that fact to have them understand this argumentation.