With regards to the bisexuality question, I think all humans have a bisexual psychology, some just veer more toward one extreme while others veer more toward the other. Gorllias, chimpanzees and bonobos all interact in both homo and heterosexual acts. From what I remember from Anthro, homosexual activity is not uncommon in hunter-gatherer societies, though it is not necessarily a strict idenity as it is in our culture.
And from an evolutionary perspective, it's quite easy to see how homosexual tennacies could develop side-by-side with heterosexual tenancies: pleasure creates postive associations, pleasure from another leads to bonding and bonds lead to increased trust. Bonds and increased trust are both integral to survival of humans in a natural setting.
In Western society homosexuality is taboo and from childhood we are taught to be discomforted by anything that can be perceived as "gay," especially amongst males. Male-male contact that isn't of an agressive nature is often questioned or condemned. Heck, I've seen little boys told not to hold hands. So I think a lot of people repress any homosexual inclinations to the point where they no longer feel them or so that they come out in wierd ways (i.e. incessant butt-slapping in a predominately homophobic sports-culture, paddling and other erotic hazing in frats).