This is something I am pretty educated about. It is natural in the sense a good amount of diseases are. The biggest biological influence is prenatal hormone exposure particularly during the 2nd trimester. This is why we see high incidences of twins having similar sexuality. It is certainly an evolutionary disadvantage and is not very genetic because it clearly does not help the passing of genes to an offspring, the mild evidence in evolution would be the gay uncle theory but has a weak evolutionary benefit. The more children a mother has the more likely they become to be gay because of the androgenic antibodies the mother can develop. I think that as we mess with our hormones and food in this artificial world we certainly spike the amount of homosexual children, another huge factor is the large societal pressure we have created and issues of inappropriate adults as well as things like drugs and pornography that stunt and intervene in sexual maturity. So yes I would say it is natural in a sense but in a sense it is not what nature intended to happen but rather a consequence of sexual characteristics being so sensitive in humans and hormones that influence development.