Here is how I understand being gay means.
To start off, no, I don't believe it's genetic to many people as it's claimed.
So we have one man and one woman, last two humans on Earth. Adam and Eve if you will. It doesn't matter if Eve lacks an arm, an eye, is fat and ugly. She is still the only woman on Earth. So for the species to survive, the adaptation part of natural selection kicks in and Adam will fk Eve.
Then let's say they have one son and two daughters. One daughter is blonde, the other is brunette. Now their son, all of a sudden has the delight to do something his father couldn't, and that is a choice based on preference. He can choose something based on something as trivial as hair color, because this opportunity exists. So guess what, he does it, because it's possible.
Obviously, as the Earth gets re-populated, there are more and more choices for everybody. So if with Adam and Eve there was no choice, and with their son and daughters had one choice, now that there are (idk) 100,000 people, there are plenty of choices to be made, on criteria such as social status, height, weight, personality etc.
But why are these choices made? Well we could argue it's because of natural selection. It is so we continue the existence of our species.
But here is the trick about humans. Out of all animals, us humans understand life on multiple planes. We live not only through a physical point of view, but from a psychological point of view too. We don't live only based on eating, having sex and owning a territory. We also live based on social status, wealth, sexual orientation and other things which you don't find in the animal kingdom on Earth. Someones social status or wealth or sexual orientation... it creates for us an emotion. We have opinions about all those things, opinions which we use in order to possibly select our partner in life, in order too continue our race.
So us humans, because we are now 7 billion people and counting, we now have the options we didn't have 200 years ago. And that is we have the option of being gay. Because we are so fucking many on this Earth, and we meet so many new people each day just by going down the street, that we literary have the comfort of not needing to fight in this life, but simply run away. Now in 2013, if we aren't deemed socially fit, it's okay, because no one is forcing us anymore to be fit. The circumstances aren't forcing us, we are too many on Earth so us being socially fit or not doesn't change anything, and since it's much more easier to lie than to tell the truth, to run than to fight, to be unfit than to be fit, we are now 'gay'.
Being gay, is just an adaptation to assure survival of one-self at a social level. Someone who wasn't able to be socially fit, someone who's brain told him "it's fucking hard and hurts to get a girl" and decided for the easier route. Many people are gay just because they are confused. Many people are still confused at 30-40 years old.
And furthermore to add, if being gay means to be attracted to the same gender, why do many people who are gay cross-dress? Why does in a gay couple of two men, one of them dresses like a woman? If being gay means being attracted to the same sex, why are they dressing and behaving like the opposite sex?
That tells me, that they find women attractive, so they would be with a woman, if they could, but they can't, so they're gay because the option exists.