I would say environment factor (personal experience) and the
puberty development (hormones and etc.), both will interact with
each other in a complex way, such that causing homosexual
behavior.
By the way, I am one of the PLU, and I have suffered same as the
others. Currently, I am still trying to change. Whether successful
or not, it is still a big question mark (???). 
Some may claim that I am under this "self-denial"
phase, condemning it hard (unable to accept self). But the truth is
that the world for PLU is too cruel (random sex, Ons, fresh look,
stagnant relationship, no long term relationship, unfaithful
partner and etc.) Sometimes we don't have the first choice, but to
pick the second choice, which is to change. 
Another question that rise is what is motive of the change: Is
it
1) To conform to the society's pressure
2) Really not happy with the current situation and fully determined
to change
Can one really change to straight? Or change to Bisexual? Or is it
just suppressing it?
But I still don't quite agree that one is born
that way, meaning that one's sexual orientation can be determined
at the age of 1. If you are PLU, did u recall that you have an
erection seeing a guy at the age of 12?
If you claim that one is born that way, what about those who are
born blinded. Can you tell whether they are straight or PLU,
because they can't see.
