Everyone could be born bisexual, and just fall into one or the other category because of the way they are raised (I don't believe this, but it is possible and would fit into some of the arguments you have made concerning homosexuality). A person could, at birth, have the ability to be sexually attracted to either sex/gender.
And I provided you with two possibilities for why we could have homosexuality. Just because you choose to ignore them, does not make them any less valid. If you wish to discuss why they wouldn't apply, that would be better.