It's abnormal in the same sense that blue eyes, blonde hair, left-handedness, O-negative blood type, and chin dimples are abnormal. It's a character trait that the large majority of the population doesn't have. But so what?
I think it's probably wrong to consider is a birth defect, any more than we'd consider those other things birth defects. Homosexuals include some of the highest-functioning people in society.
I think it could even be understood to be an evolutionary advantage in an indirect way. It at least appears that homosexuality is more likely to manifest in contexts of denser populations. That's something that shows up with other kinds of latent genes in nature -- like grasshoppers can have certain genes switched on when raised in crowded settings that cause them to develop into locusts that then swarm. Well, maybe when lots of humans are living in an area, it benefits society if a larger minority of them start pairing off in same-sex relationships, with the idea that will somewhat decrease birth rates, while allowing society to invest more resources into fewer offspring (e.g., the gay aunts or uncles who are more likely to be selfless resources for breeding siblings).