The hypotheses about whether homosexuality could have an evolutionary explanation are very unconvincing IMO. The 'benevolent uncle' theory and the 'sexy sisters' hypothesis seem far-fetched.
Similarly the idea that something is 'natural' because it happens in non-human animals is a strange argument. Animals commonly get up to all sorts of antics we rightly condemn in humans. Actually homosexuality is very rare in the natural world. Bi-sexuality is common where beta-males have no access to females. Some domesticated sheep are a notable exception.
I think it is a mistake to try to provide biological justification for homosexuality. Even if it is simply a lifestyle choice (and I'm sure it isn't that) there is no reason to condemn people for their sexuality as long as it is consensual.