Saying that homosexuality is \'unnatural\', ie outside of the bounds of the subset of behaviour that would occur in a controlled group of human beings with no external input (testing the human \'animal\' as it were) is unsupportable. Empirical evidence of societies that existed outside of the bounds of a moral perspective that homosexuality is wrong disprove it.
So the condemnation of homosexuality is always based on a moral perspective. Those who oppose homosexuality can never say that it is only a symptom of abnormality (due to the large numbers of historical homosexuals who were, in all other respects, functional and ordinary). But they will fall back on the position that it is a personal choice, and that it is a choice for a morally wrong mode of behaviour.
So you have to inquire as to the basis for that moral belief (ie what is the set of beliefs giving rise to the comparison) and test how infallible that moral code is or has been in the past.
Take christianity for example. It appears to be a tennant of that faith that homosexuality is wrong. So, you have to ask "has christianity always been right?". I however think that the better question is "has christianity always been consistent?".
Christians believe that their moral beliefs come straight from god. That, by its nature, should be an immutable truth. God doesn\'t change his mind or the interpretation of his words. So, if thats true, then christianity should have retained a single immutable unchanged set of values and beliefs throughout the entirity of its history. After all, they\'re never wrong, and its gods own words. Sadly, that isn\'t the case. It seems that the tenants of the christian faith have changed and developed over time (eg women are no longer seen as the progenitors of original sin who had to cover their heads in gods church), albeit not with respect to homosexuality.
Its enough to say that the christian moral code is variable and therefore serves as a poor basis for judging a type of behaviour that predates the religion itself. Not that christianity is bad, I\'m a christian myself, but rather that christians should realise when they have been handed a moral compass and programmed to use it. I mean really, how many people here came up with the idea that homosexuality is wrong all on their own?
So, if homosexuality occurs when there is no forced moral taboo on it, it is scientifically within the group of \'normal human behaviours\'.
And to validly compare a particular behaviour to a moral code and to demarcate that behaviour as an absolute wrong, that moral code must have the qualities of consistency and objective universal validity.
Homosexuality is not a choice. No sexuality is a conscience choice. You choose who to engage in sex acts with but not to whom you are innately attracted to. You do not decide to be attracted to a handsome specimen and then decide to not be attracted to some morbidly obese one; you just are or are not attracted (love at first sight?). If one could decide who really turns them on then you should be able to enjoy sex or at least be as equally excited by the prospect of sex with anyone, regardless of appearance; which we know* is absolutely not the case.