I think that our biology (our nature) helps determine whether we are prone to developing this way or that. I think that people's biology helps raise the percentages....like this male has a 90% at being gay or this female has a 40% chance. The people that are 50% tend to be bisexual in my opinion because their environment gives them positive and negative experiences with both male and female people. On that note, I think that people will gain experiences with both genders that will seat an impression of what they will desire in a partner later in life, but I doubt that it has that much influence. It's more of a slight deciding factor. So in short, nature is the bulk, but nurture does contribute (however little).
Here is a video posted on youtube that may help answer some questions. Its defintelty of a mature nature and should not be viewed at work or by prudes or by anyone offended by animals behaving "unnaturally." I'm hoping it will put to rest the fact that the folowing topics do in fact happen in nature, and not just by "disgusting humans." Masturbation (both male and female). Fisting. Licking (male/female) Inter-species sex. Inter-species masturbation. Gay/Lesbian sex (only "lesbian" is shown in this vid, but I have seen a gay donkey going at it (yes, anal and to the point of ejaculation.))
These are not tamed animals, nor at a zoo. Just animals having fun. like we do.
I like to find vids like this just to prove to the fundies out there that being GBLTGPQ??? Is perfectly normal. http://youtu.be/5vQ-bqC7wEw