First of all, pederastic relationships are not at all the same as homosexual relationships between people of age. In ancient Greece, homosexuality between adult males was condemned. Your theory that the range is from 100% to nearly 0% is simply wrong.

I think we say this because we have stronger taboos against one than another. Keep in mind that women were considered to be at age of marriage at not much older, so... Also it is simply not true that homosexuality between adult males was condemned. At most homosexuality between adult male equals was condemned and the reason probably had to do with the association of anal sex with blurring gender lines (Greek pederastic relationships if we believe Aristotle, made penetrative sex strictly off-limits for this reason).

Keep in mind also that "adult" is another inherently cultural concept..... It happens at different ages in different societies. When does one become an adult? Age of 14? 16? 12? 18? 21? Something else? There is no biological line there either. Indeed we've known since van Gennep that puberty and adolescence are distinct and may not even overlap in any given culture for a specific individual.

But hey, if we take Plutarch's word for it wife-swapping in Sparta was the way adult male equals were supposed to... um... build connections.....

But it's not limited to Greece. When you look at cases where ritual male-male fellatio is practiced, some of these places have it going well into adult-hood rather than one-time becoming a man. I don't know that you can call ritual homosexual contact "not significant." I would certainly call it significant.

But even where we get to attraction, there are some basic problems which have to do with getting past the feeble nature of the senses.

We can't directly sense whether someone is male or female, what social class they belong to, what kind of personality they have. So we process social cues. We see someone from behind "nice long hair, skirt, must be an attractive woman." We walk by, check her out and are horrified to see it is a bearded hippie. Everyone has had some sort of experience like this, and in our culture it provokes immediate negative emotions.

Sometimes these emotions of shock and dismay (and even confusion) lead to tragedy. In one case in Denver not too long ago (see http://www.thedenverchannel.com/news/19121127/detail.html) a man murdered his transgendered girlfriend after discovering she had previously been a man. She had given him a blowjob the day before, and evidently he couldn't deal with the confusion over whether he had been given a blowjob by a man or a woman. This is a purely cognitive process, not one based on biology. If it were based in biology, I suppose we could treat her as the perpetrator of non-consensual sex-acts by failing to disclose the inherent nature of the act.... but I don't think think that works.

So sexual attraction, and reactions to what social category someone is in (and this includes gender) are fundamentally based on cognitive processes. There is no way around this. Cognition and social instincts, rather than the inherent nature of the object of sexual desires, determines who we are attracted to.

At most biological underpinnings of sexual orientation in our society affect these social instincts and cognitive aspects. They simply can't reach the underlying nature of the, if you will, archetype of attraction. (Using "archetype" here to mean the image we have that we expect to resemble the person we are attracted to.)