I think it’s a very reasonable argument. It makes the point that homosexuality, despite the claims of many organisations that object to it, is natural, because it happens in nature. It also makes the point that it’s not a choice. Most other species are not in full conscious control of their sexual impulses and cannot opt to abstain the way some humans can. They are not making an active choice about their sexual orientation, which in turn implies that the direction of action is not a choice for humans.
The funniest attempt at a counter-argument I have heard was from two elderly Jehovah’s Witnesses called Dulcie and Pauline who doorstepped me and a friend while we were writing sketches. They gave us a long harangue about homosexuality being unnatural, after which we pointed out that many animals also engage in homosexual sex. To which they replied “But that’s because of humans, isn’t it?”
My writing partner was so angry that he went back to the keyboard and immortalised our conversation in an 8-page skit in which Dulcie and Pauline (who kept introducing themselves, but alternating who got to say “I’m Dulcie and this is Pauline.”) were ultimately seen off by a small child who calmly called them on their bullshit over and over.