As someone else on here argued I think that many people choose to be straight all the time and deny same sex attraction. I'm not saying that it's wrong. It's just a fact that people deny homosexual attractions all the time. One could argue it isn't the best choice, but I don't deny that they're the ones ultimately responsible for making that choice and therefore for deciding the morality behind that choice as well. Where as other might decide to chose to deny heterosexual feelings for moral reasons or as a matter of taste or preference, what have you. A common theme among many lesbians is that they wanted to have deeper bonds with other women and felt it was a more moral choice to have same sex attraction for that reason. In our society women are taught to compete with each other rather than combine their energies. Choosing to be straight might be harder then because it might seem like a betrayal to the conviction you had in the first place, or a betrayal to yourself and your gender.
If homosexuality were the norm, I think someone could choose to be heterosexual as well. It might be more difficult because there is already a divide between men and women that makes relationships harder, and it would probably be even greater if the majority was gay. But some people might feel a moral need to bridge the gap between men and women in such a society where inter-gender love was not the norm, the same way women often feel that they need to bridge the gap between women in a society that places little value on women's relationships. Then choosing to be gay again might happen when they feel like they've accomplished what they set out to accomplish. Or maybe they just realized that they didn't like the differences between men and women and ultimately didn't want to bridge that gap.
I definitely think that choosing heterosexuality is a boring choice for someone who isn't really thinking about things, about what they care about, what turns them on, etc. If all they're doing is letting society dictate to them what they should do. That isn't to say that heterosexuality itself is boring, it's just boring to choose your sexuality for no real reason and without passion or attraction or conviction. I think it's an acceptable choice and many people are happy with that choice, but there are also those people who chose to be straight who deep down probably should be asking if they really want to be. I see the notion of being gay by choice empowering because it says that straight people don't need to be straight if they don't want to be. It isn't saying that being straight is wrong, but it's saying that you don't have to be "born that way" to start exploring same sex feelings and attractions. It's saying that you can be more deterministic about yourself and whom you want to become, and it gives you the authority you need to be attracted to the people whom you choose to be.