Scientific fact isn't necessary - it's about logic. Just read this:
Where is research or scientific proof that people are born gay? I've always heard this, but have yet to see scientific fact. I have seen scientific fact of people being born black.
There is a difference between preferences and choices.
A) Most of us have straight preferences, and choose to have straight relationships.
B) Some have gay preferences and choose to have gay relationships.
C) Some have gay preferences and choose to have straight relationships (for societal/family acceptance, etc)
D) Not many have straight preferences, and choose to have straight relationships.
E) Some people have gay and straight preferences, and choose to have straight relationships.
F) Some people have gay and straight preferences, and choose to have gay relationships.
You can choose actions, not preference (I don't choose my favorite color, I choose the color clothes I wear). If someone has a choice, and has equal opportunity to be happy either way, they must fall under category E or F, therefore are bisexual.
A straight person falls under A or D, then a gay person must fall under B or C.