I'm no biologist but I do think that there are reasons why homosexuality might be considered normal behavior.
First, it has been observed on many different animal species and certainly has existed among humans since as far back as the historical record goes. So evolution has not caused it to go extinct.
Yes, it has been observed with other species, this has been documented and thought of as a dominance behavior of these animals, but note, these animals are not only gay persay, they also have sexual relationships with females.
And yes, homosexual sex has also occurred among humans and was even accepted in societies such as in the Roman Empire, where it was OK for Roman men to have homosexual sex with another man, if the man was also his slave and if it was the Roman man screwing the slave, but NOT the other way around....this too was a sign of dominance, and accepted.
I am not denying homosexual desires have existed among humans forever, i'm just not a believer that one is born with them for any scientific or survival reason.
Second, perhaps it is a response to overpopulation, a way of protecting a species that has overcrowded it's habitat.