Kind of a (very) late reply, but better late than never. Lots of things you posted are wrong.
1) Human beings are, in fact, pretty damn bisexual, your silly comments about the erastes-eromenos relationship aside (also, that has been recorded to have been enjoyed by many eromenos!). Many, many historical have had prolific expressions of male-male homosexuality, which is generally expressed as either mentor-student relationship or a relationship between two masculine warriors. There are major mythological examples of these: Gilgamesh and Enkidu, Achilles and Patroclus, etc. Homosexual behavior between peers was a part of Greek military doctrine. Homosexual relations between masculine braves in North America were prolific and there was no view of non-masculinity on the part of either partner. Nambikwaran society has universalized and more acceptable (i.e. fucking in the middle of camp is okay, unlike for heterosexual relations) sexual relations between brothers-in-law. The Origins and Role of Same-Sex Relations in Human Societies has loads and loads of example of male homosexual relations that were peer-based and near-universal.
2) I have no idea where you got the "15% of men have experienced some same sex attraction." I'm pretty sure it's more than that! The Kinsey Reports (and successive research to clarify the data based on problems with the original data acquisition methods) put the number of men who have achieved orgasm via contact with another man at 37%, and women who've done the same with another woman at 13%. That's more than 15%! And that's actual sex acts, in the USA, decades ago!
3) If human sexuality is innate, we would expect it to mirror other mammals and primates, and guess what: they engage in loads of homosexual acts. Chimpanzees, bonobo chimpanzees, baboons, gorillas - the prolific nature of their homosexual behavior is well-documented.
4) Delusions of Gender deals with a whole lot of research that shows that: the sexes have no difference in social skills; some societies have statistically identical std devs for female and male ability at mathematics; newborns have no difference in preference for systemizing versus empathizing stimulus based on sex; and there is a major negative psychological effect from gender stereotyping on relevant skills. If Delusions of Gender fails to conclusively disprove any sex-based difference in, say, math skills, then it certainly provides an excellent argument that we should pretend that it has.
The source he cites is A World on the Wane, or Tristes Tropiques in the original French.
It can be found here:
http://www.archive.org/details/tristestropiques000177mbp
The quote from the book itself (p. 307): "The Nambikwara have, however, another way of resolving the problem, and that is by homosexual relations or, as they call them, tamindige kihandige: 'the loving lie'. These relations, common among the younger men, are carried on with a publicity uncommon in the case of more normal relations. The partners do not go off into the bush, as they would with a partner of the opposite sex, but get down to it beside the camp-fire, much to the amusement oftheir neighbours. The incident provokes a joke or two, on the quiet, the relations in question being regarded as childishness and of no serious account. It remains doubtful whether these exercises are carried to the point of complete satisfaction or whether, like much that goes on between husbands and wives among the Nambikwara, they are limited to sentimental out pourings and a certain amount of erotic fore-play.
Homosexual relations are only allowed between adolescent boys who stand to one another in the relations of crossed cousins cases, that is to say, in which one partner would normally marry the other's sister and is taking her brother as a provisional substitute. Whenever I asked an Indian about a relationship of this sort, the answer was always the same: 'They are two cousins (or brothers-in-law) who make love together.' Even when fully grown, the brothers-in-law are still very free in their ways, and it is not unusual to see two or three men, all married and the fathers of children, walking round in die evening with their arms round one another's waists."