This reference to "the norm" is a bit confusing. Rather than look at it in statistical terms, which is essentially how it's being portrayed, it's probably more useful to think of a behaviour as being within accepted social limits. Now social definitions of acceptable behaviour arise from the cultures of societies so it's not that helpful to talk about worldwide "norms" or "norms" through history. We define what's normal by defining what's abnormal or deviant. Those terms are now loaded but I'm using them dispassionately. We decide what's deviant socially and usually it's those in positions of power who decide it for us. Religion is a powerful means of defining deviant behaviour and has been throughout history and across cultures. Look at the efforts by some groups in various societies to push Sharia as the legal code of their country. They want to remove secular law and replace it with religious law. That would radically re-define deviant and non-deviant behaviour and it wouldn't just be about sexual matters.
Shogun has pointed out that homosexuality isn't seen as a mental illness any longer by the medical community, those gatekeepers of medical knowledge. Now being as that's a clinical decision I would think that the APA have done a lot of work on that. I don't know this but I wouldn't be surprised if the APA decided more harm befell people when homosexuality was considered to be an illness than when it was taken out of the DSM.
AB mentioned chimps. Murderous little bastards they are. They hunt in packs and tear monkeys apart and eat them. That's normal behaviour for a chimp. Bonobos, no, they're not as aggressive as chimps. Chimps hunt in packs and kill monkeys - is that "wrong"? No, it's not "wrong", it's perfectly natural behaviour for chimps. It surprises us because most of us have only seen chimps making funny faces to their keepers in the zoo. We, who get out meat from shops which in turn get their meat from slaughterhouses where animals are slaughtered, turn our noses up and accuse chimps - as I did - of being "murderous". It's ridiculous to do so, it's "nature red in tooth and claw."
http://www-rcf.usc.edu/~stanford/chimphunt.html
Some societies accept homosexuality more easily than others. There are social reasons for that (not that I know what they are). Some even accept situational homosexuality.
http://www.colorq.org/Articles/article.aspx?d=QHistory&x=parallels
Obviously I don't know why homosexuals are oriented towards their own gender. However I do know about sexual orientation and impulse, being a human being makes me an expert in that. Growing up I knew that homosexuality was defined in my culture at that time as abnormal and in fact criminal when acted upon even between consenting adults. If homosexuality is chosen as some assert, why would you choose a sexuality that could see you imprisoned for indulging in it with another adult? Why would you choose to become part of a despised minority? It seems to me that sexuality is a lot like race, you don't get a choice.