My question for everyone who says it's genetic: what's your evidence?
Since there is currently no evidence for it either way, I tend toward believing that it's the same as all your other preferences and aspects of your personality: almost entirely nurture.
Keep in mind two things from this point on:
1. I AM NOT SAYING THAT ANY SEXUALITY IS WORSE, OR LESS, THAN ANY OTHER. Love is love, and love is good. (Obviously I'm talking about mutually consenting love here.)
2. I am also not saying that it's a choice, or that it's simply a matter of "My parents told me not to be gay, so I grew up straight! Simple as." Stuff doesn't work that way. It's subconscious.
Your sexual orientation is like your favorite color. You can't one day wake up and say "I think I will choose to feel that red is the prettiest color." Nor can your opinion be swayed just by your parents telling you that you must believe that red is the prettiest. You feel what you feel, and that's that. However, you're not born with a favorite color; you develop one as a result of your experiences. It's also possible for it to change over time, though it doesn't have to. Then there are some people who would rather not have a favorite, they just treat everything as situational ("I like this green, but usually I like yellow best.").
In this discussion you'll hear a lot of people saying "But my family hates gay people! How could I be gay if what you say is true?" Obviously humans aren't robots. We don't get receive simple, straightforward programming and continue on with simple, straightforward lives. It's much more subtle than that. For one thing, constant pressure to not feel a certain way often leads you to feel that way even stronger. (Example: Victorians finding ankles arousing.) Also, what your parents say to you means very little in the grand scheme of things.
Here are three examples from real life (all friends of mine):
1. A woman is raped. She is not too keen on penises from then on, and so she decides to seek out women only from that point.
2. A boy grows up with very low self-esteem. Soon, he starts paying attention to other boys whom he admires and wishes to be like. At a certain point, this admiration becomes sexual attraction.
3. A woman is going about her life, contentedly dating only men, when suddenly she meets another woman and unexpectedly falls completely head over heels for her. They get married and live happily ever after (for ten years so far).
These examples are all pretty straightforward, though. It's much more complicated for the majority of other people, and you will most likely have no idea why your sexuality is the way it is. Either it happened before you were conscious of what was going on around you, or it happened so gradually that you couldn't possibly trace it all back to its various different sources.
Another important thing to mention is that no one can force anyone to change their feelings about anything unless they really, truly want to change themselves (in which case they probably already have). Unless you brainwash them, which is pretty much evil. All that nonsense about camps trying to turn people straight is just that: nonsense. Same with the belief that growing up in a culture that accepts homosexuality will turn you gay.
However, people like to believe that they are who they are, and they always have been and always will be. This is not true. Pretty much everything you consider to be "you" is something that could very easily be different if you had grown up differently. Your personality is not you. Accept this and you will be a happier person.
I have to mention this because many people find that their sexuality does change, and they usually react pretty poorly. They figure that if their sexuality has changed, it means that they must have been wrong about themselves all along and that they should reevaluate their entire lives up until that point. It doesn't mean that you're a different person, it doesn't mean that you're a traitor, and it doesn't mean that anything is wrong with you in any way. It's just life.
