The geneticict's reportedly have found a link between certain genes and sexual preference but I don't think it is that simple. Rather, I think it may be a combination of genetic makeup, how we are socialized during childhood, and life experiences. I don't think we fully understand why some people chose to be homosexual and others chose to be heterosexual.

The truth is that science doesn't really know why some people are gay and some are straight.  Some people find even the search for a cause offensive because they feel that looking for a cause means there is something wrong with being gay.  However, science has sought to understand the reason why heterosexual couples are attracted to one another, so it makes sense that science would seek to understand why homosexual couples are attracted to one another.  Some claim that genetics is a factor while others say it is all environmental.  In truth, it may be both.  While there is no conclusive evidence that genetics is a factor, we do know that sometimes the environment is a factor.  Experimenting in the early teens, like high school, is becoming more and more common, especially among girls.  

It is quite common for young people to engage in sexual experimentation.  Also, if the opposite sex is not available sometimes people will take advantage of what is.  This is what happens in prisons.  I think it is more likely that you are still forming your sexual identity, and it may not be so much that you are no longer interested in girls as you are no longer interested in anyone but this boyfriend.  Good for you, to have such a meaningful relationship!

Statements of politicians, in my opinion, have little to do with whether something is right or wrong (I believe post 4 is intended as sarcasm, but the point still stands.) A number of studies, one of which I link to below, have suggested that genetics play a major, even a determinative role, in one's sexual orientation. Random developmental factors, many of which may occur while a child is in the womb, do as well. 

http://www.sciencedaily.com/releases/2008/06/080628205430.htm

As the first response indicates, sexual preference occurs on a continuum, and it is not a simple matter of locating a "gay gene," but the scientific community seems to be convinced that our sexuality is linked to genetic factors. 

LIke the nature/nurture argument, I think this one is impossible to answer for sure. It certainly seems like there is a genetic component, but can we really be sure it's that or just the way someone reacts to the environment in which they are raised? I doubt if science will be able to answer that question one way or the other.

I definitely don't think that it is just a casual decision somebody makes. There are powerful forces at work shaping the person.

I really think that your way of phrasing this is a bit judgemental.  The idea that gay people are in some way imbalanced is certainly going to sound offensive to some people.  I believe that sexuality occurs on a continuum.  There are people who are 100% straight and those who are 100% gay and those who are in between.  I believe we are born that way.  Perhaps if you are in the middle you can experiment and have your social conditioning push you one way or the other.  But there is an underlying inborn aspect to our sexuality that is very strong.  To say that one kind of sexuality is the result of imbalance is (in my opinion) wrong.