The problem I have with that is multi-faceted.
First, there's a 54% concordance rate between twins having the same sexual orientation. So while social and psychological factors obviously play a role, it's clear your biology accounts for a large proportion of the end result.
This actually plays perfectly into what I'm saying.
In addition, offspring can be born gay because the mother forms an allergic reaction to the H-Y antigen after multiple male births. So if you were a male child who was the last in a line of male births, you are "at higher risk" of being born gay (something like 30% of gay men are thought to be born due to this). Conversely, excessively high estrogen levels in the mother, or great stress during the prenatal period has been show to have the same effect for female offspring.
This I've never heard of at all, ever, anywhere.
Second, pose the question to any heterosexual male of when "he knew he was straight". Repeat that a very large number of times and you will find it happens at a very young age, and in less than 2% of the cases, never changes. It's certainly not something they consciously got to decide upon.
This also actually plays perfectly into what I'm saying. It's not a choice for someone, they already have attraction to both regardless if they want to admit it or not. Whatever factors (social, psychological, etc.) make them "embrace" either one side only or both sides is up to them to pinpoint. Choosing 'not' to do something isn't the same as choosing 'to' do something.
If we were born bisexual as you pose, then Freud's theories or the failed practices of Dr. John Money would have held water. I am not discounting that a person's orientation may be influenced by social factors (like in BEM's exotic becomes erotic model), or that positive and negative psychological thoughts help crystallize things. But I do think many people mistake how they label themselves and their behaviors (sexual identity) for their inner compass of who they are actually capable of loving (sexual orientation).
What sexual side people want to show is definitely influenced by social factors. Remember to most societies, anything gay is bad. People will more times than not choose to bury any and all types of homosexual thoughts and behaviors out of fear of being ridiculed (or something greater like losing a job, relationship with a family member, friend, etc.) You're saying the social factors make someone into either being straight or gay. I'm saying that the social factors make someone either show or hide one sexuality or the other.