[headsplice]

This is a false dichotomy. Very, very little of our psychological makeup (and, more particularly, our actions) is completely determined by our genetic sequencing. A better way to say it is that our genotypes set a range of possible outcomes for phenotypes, which are then more concretely manifested after interaction with other people.
In Kinsey's terms, you aren't 100% gay or 100% straight. Most people fall somewhere within the range of bisexual, either less or more so. So, if you have a stronger genetic pre-disposition to being gay, it's more likely that you will be attracted to the same sex, but it is by no means a guarantee.
Of course, by no means does that imply that your sexuality is a 'choice' (defined as: actively deciding, "I think I'll dig dudes today, because I'm tired of chicks pissing me off"). You are still attracted to who you're attracted to and trying to change that makes you a little bit crazy (have you ever seen the people who have 'gone straight'? That's clearly an abuse of the god-given gifts of repression and denial). Generally speaking, the factors that lead up to that 'choice' are invariably out of your control (like being molested, or having a strongly pair-bonded set of hetero-parents) and those are the factors that end up influencing your sexuality.