A lot of people still get hung up on homosexuality because they take an overly reductionist perspective. "Mans don't have pipes to make babies so mans weren't meant to go together, so that is illogical. BZZ. INCORRECT BEHAVIOR."
First, putting aside the fact that being gay is not actually a choice to behave "illogically" (and thus by the naturalistic argument, being gay is not an "error" because, well, it's natural), that kind of reductionist thinking leads down a hall of mirrors. One could start arguing "but humans should not be in aircraft, they did not evolve for the function of flying."
Second, as others have brought up such examples, reductionist naturalism can't cope with people who mate but don't have offspring on purpose, people who remain celibate, etc.
In point of fact, there ARE those who believe that it is "wrong" for people to not all pair up and have children. There are those who do say it's wrong for married couples to remain childless.
In those cases, they are a small minority so you don't hear that argument breaking out into huge fights in culture. Most people can see that human behavior cannot be reduced to that level, so they take for granted that there are celibate people, childless families, etc.
I think there may be a connection there however.For instance; people who argue that childlessness is "wrong" even immoral, tend to be too concerned with how others live their life, and disturbed, unsettled, by the fact that another couple is not behaving in a normative fashion. People are insecure things. Sadly, when many see someone behaving in a way that is not, to them, normative, their first reaction is not to wonder "what new behavior am I unaware of" but to think "something is WRONG with them, nobody who behaves correctly would do that." (This in itself seems as if it can be a matter of education. It is possible for someone to learn that they cannot judge other behaviors that easily, and this is part of being open minded.)
In the case of The Gay, it is highly non-normative behavior in cultures that suffer from a lot of homophobia and censoring of the topic. Therefore, it is bound to unsettle or confuse people to a great degree and trigger the impulse to explain it away as somehow wrong. It's true that this doesn't necessarily mean each individual who is confounded by The Gay will transform their confusion into hatred. It's just that it's easy to happen with the human animal.
Someone asked early in the thread about the "phobic" part of homophobia, since so many people seem to just outright hate gay people. But that's how it works. The hatred is generally a mechanism to cover up fear and insecurity. Homosexuality threatens the social foundations of people who are insecure about those foundations. Look at how common it is for an extroverted, extremely hateful homophobe to be outed as gay himself. Classic projection of personal confusion and insecurity onto other people / another group.
