Nature.
I think its in our instincts to want to 'survive' and 'procreate' so we may live on through generations. We want our species to last, so from that standpoint, I feel it is a nature thing. And by nature, I believe that when a population gets too high, homosexual activity increases to cause a form of balance and population control. That doesn't really cover bisexual preferences or whatever there is out there though...
You might also argue that the only reason why homosexual population goes up as theres more of a population density is because more people = you see more of the uncommon things as well due to so many people.
So, I don't know a heck of a lot about the science and biology standpoint surrounding those theories, but its an idea none the less.
Either way, I'm going to go with nature.