You could make an assumption here based on the time period, being that of time's of war. In time's of war, when there are very few women about, I think a lot of men subjected themselves to other manors of sexual pleasure, to resolve that *need*.
I disagree with Source's statement above regarding all humans by default being bisexual. I personally have no sexual interest with men, I kid and joke around, but for the 'fun of it'. Why not, there is no reason not to really. I'm definitely not modest in that area.. if I can freak someone out I will do it.
Look up the Kinsey reports. It doesn't go as far to say that everyone is bisexual, but most people are bisexual at any one point in their lives. Oddly enough, women tend to be more often bisexual than men.
Oddly enough? Ever look at your penis, it's kind of a turn off. :p
I'll have to look them up. I don't know what constitutes as bisexuality though. We could always go the Ron White description. Regardless, I think its too far to say 'almost everyone is'. A lot has to do with cultural and society impacts on life. American culture is less modest, than say, Pacific Island nations, Europe is very not-modest with a lot of things. Islamic countries are very modest with everything.
Specifically looking at the Samoan culture, sexuality in general is shielded, yet there are open transsexuals, which is built directly into the culture on the basis of needing either men or women in the household, in which if there are less women, a boy is then raised as a women to do women like duties. At the same time sexual education is greatly suppressed due to it being a 'no-no' topic because of the modesty of the culture which (from what I've observed) causes kids to 'fool around with each other'', more specifically with their own gender, due to the vast separation of men and women. (Mind you, this is one of the few cultures where men and women are fairly equal in status).
Anyway, those are my observations, which doesn't necessarily make them true.. just compiled from what I've seen and heard.. and.. not that it matters, but adultery seems common here..
Edit:
(I think what I was getting at with that was, do sexual experiences make you bisexual, by fooling around with the same sex out of curiosity, or actually physically having attraction to another person of the same gender?)