this makes most sense in my mind, that we all have a genetic predisposition to be bisexual, just because we can find love to anyone [i think its very important to remember there is a VERY large difference between sexual attraction and LOVE]
Love is definitely an ambiguous term, especially in western culture since many sorts of feelings get lumped into it. (The Greek had more terms, with more nuance, to describe various facets of love.)
And yes, we can physically engage in sexual play with either physical gender.
But it shouldn't be surprising that such a large segment of the population is het, due to the reproductive needs. There's definitely a large slant towards that...
... I really do not think all of sexual preference and "love" is nurture. Biology tends to transcend training in areas like that; you can be trained and pushed and yet the innate still surges out. It can only be suppressed.
That being said, I think it's pretty clear that social and religious values greatly determine the ease of expression of certain preferences, and often if we attach sexual stigma to something, it seems to create more issues than fixes.
*fact* - i have 5 sisters, and 1 brother.
*fact* - i am straight...but not entirely

theres room for variation at times
*fact* - i believe that it was growing up around girls that has made me quite effeminate, and a Metrosexual.
Meh. It can, to a degree, simply because more "effeminate" behaviors and mentalities will be picked up and you wouldn't have been exposed to as many "masculine" behaviors if you would have grown up in a largely male environment, but it doesn't really much change your internal preferences or where you might lean if the opportunities presented themselves.
Funny, my one metrosexual friend also drives a jeep and enjoys doing overtly masculine things despite fully admitting his metrosexuality. As an INFP male, he is quite a mix.