Gender preference may not seem like a conscious choice for many people, but I believe that it is more likely than being born with a sexual preference. Gay, Straight or Bi, it's natural for us to find individuals attractive rather than an entire gender as a whole. To say a person is gay seems like a massive generalisation to me, as it implies that the person is attracted to the entire sex they are a part of, which is never the case. I've known gay men who have had attractions to females and gay women who have had attractions to men, whether they have resisted those feelings or indulged them.
When it comes to pro-creation, opposite genders are required to have sex as a matter of our species survival. When it comes to sex for the purpose of pleasure there are no limits, other than the limits we ourselves put upon it. For all we know, gay people are necessary in nature as a way to slow our population growth and are natures way of balancing out the ecosystem. Nobody can deny that there doesn't seem to be an intelligence behind nature, guiding it here and there and our species is getting so large that eventually our ecosystem will not be able to support it.
How many straight men in relationships have had affairs with men and the same with women and other women? It's in the papers and on TV every day and it's accepted as taboo by everyone, without question, thought or research. Why is that? Because denial brings comfort to many and they don't 'want' to know the truth, as they find it threatening.
Our species has bisexual attributes and always has, you only have to research the Romans and Greeks to see that fact. The same with Incest, it's the mistaken assumption of religious people who have always believed they have moral superiority, who have twisted the way people think about the natural practices of our race and have made much of it seem unnatural. This doesn't mean I would feel comfortable doing either, but then I was raised in a society that has been manipulated by the religious to believe it's wrong.
When people try to live their lives as one thing and one thing only, I think they are most likely living in a perpetual state of denial and are never going to be truly happy, as they are closed-minded in that respect. The happiness they feel is artificial and one day that will become clear to them and by then it may simply lead to regret. I think it comes down to the individual and how deep their denial is and how far they're willing to go to resist their true nature.
For the record, I'm not gay and do not consider myself bisexual, but that doesn't mean I am not open to the possibility that some day I may meet someone who changes that. :o)