To start the ball rolling - let's look hard at the underlying assumption of your post: that sexual orientation is predetermined. That I think is a fundamental error - I think in reality a lot of factors go in to determined what in practice a person finding sexually attractive.

3 examples spring to mind:

1) There is a fetishism associated with leather. Since that is a manufactured product, we can fairly easily see that it is result of learning somehow to be sexually aroused by leather.

2) In parts of the world today, and in the West a few centuries ago, fat women were perceived as more attractive. These days the preference is the opposite.

3) The story of Sparta is instructive: ALL boys were inducted into the army at a young age and part of their lifestyle there was to have homosexual sex with their peers. At the age of 25 or so they were then pushed into marriage - and not surprisingly struggled to get aroused by their wives - it's probably the case that this is one of the reasons why Sparta declined as a military power in the long term.

I'm not denying that there is some predetermination - I'm not sure. But what I am sure is that nurture also plays a substantial role.