This is not about whether it is OK to be gay, or whether being gay is a choice (which is entirely different from whether or not it is OK to be gay). It is about whether people use fallacies in arguments about whether gay people have a choice.
And they do. They tend to reduce the concept of choice to something that must be doable instantaneously and without effort, and must be something that every single person consciously chose and not something one could consciously choose if one wanted to but which most people don't bother with. They do this by saying things like: When did you choose to be straight? Or saying things like why would someone choose to be gay with all the hate and prejudice.
It is generally possible to make oneself like something (or conversely hate something) if one puts in the time and the effort trying to get it to happen. That doesn't mean that most people actively make that choice for any given particular thing that is liked or disliked.
Studies show genetic correlation for sexual orientation but it is not 100% within twin studies. Saying this proves it's not a choice invites people to make the same assumption about anything and everything that shows even the tiniest genetic correlation. The possibility of something influencing someone in one direction or the other but it still being the person's choice is suggested away by the sorts of arguments used to say being gay is not a choice.
Just from what I've seen it seems like most people who argue it's not a choice are more motivated by politics. Though the same usually goes for people who say it is a choice.
Why do we need to prove it's not a choice? Why can't people just do what they want whether that's being gay or not?
That doesn't mean that gay people do not have a choice. Whether people use fallacious arguments to defend that position is completely irrelevant to the facts, and in fact, to think that an opponent's fallacious argument proves your position is called the fallacy fallacy. Gay people do not have a choice, but that would be the case whether people were committing fallacies or not. I know this is a debating website, but try not to get too hung up on fallacies — avoid them in your arguments, and watch out for them in your opponents', but just ‘sniping’ by pointing out fallacies is unproductive and boring.
People who claim that gay is a choice (ignoring the common sense that blatantly contradicts this position, as well as recent scientific findings) generally do so as an argument against the legalization of gay marriage. There is a fallacy in using this as an argument at all. Society has no business where two consenting adults in a bedroom are concerned. Even if everyone who is gay chooses that lifestyle, that is their personal choice and should be respected by society. (I do not believe it is a choice, but that is irrelevant to the question.)