Hmm...

[Paco]

It's possible that there would be certain effects by certain genes that would make a person more prone, when exposed to certain environmental circumstances, to be attracted to the same sex.

This is the most probable of any explanations I've come across.

My post from another forum:
As far as comparing us to animals, we're just as into instant gratification as animals are. Infact humans are the poster child of excessive gratification behaviours, in spite of our knowing better.

Capacity plays into it.

We have the unique ability for the most part of being able to take a stand offish approach and be selective about our behaviour unlike, for example, a dog.

That doesn't change the fact that the drive is still there.

And the degree in which we can deal with those compulsions is a luxury not everyone has either. There are a slew of mental conditions that people are aware they have, but lack the ability to do anything about it.