It amazes me that people still think that people choose to be gay, lesbian, or bisexual... and I suspect they even believe transgender people chose that life, too. After so many people have told them otherwise, they still insist on believing it's a choice. Why is that? WHY?! Why are they so fucking stubborn?
They'd rather believe that it's a choice than listen to the people who would know better than them, you know, the people who are LGBT and/or people who study LGBT people, and what makes us tick.
So many scientists, psychiatrists, psychologists, and the like, have explained what they believe determines sexual orientation, and the information is available all over the internet, so I'm not even going to bother searching for you. I'll tell you it's out there, and all you need to do is look it up. I will tell you that what you need to do is avoid using religious websites to get any sort of information about human sexuality.
In order to make a choice one needs to have a desire to do so. When you open the refrigerator and see a Coke and a Pepsi sitting there, you will make the decision to drink one or the other based on your preference for one over the other. You may reach for the Coke because you prefer its taste over Pepsi. You didn't choose to prefer Coke. You just happened to like it more. Someone else might have grabbed the Pepsi instead. And another someone else might have grabbed either one. None of them made the decision as to which one they liked. They did, however, decide which one to drink based on that preference. So, if you insist on claiming there's a choice involved, that's about the extent of the choice: The choice to pursue what one prefers.
If the soda (pop?) analogy doesn't work for you because, well, you kind of have to taste them both before you know which one your prefer, we can go with colors, instead. In the color analogy, you've just moved into a new house and there are two bedrooms from which you can decide to claim as your own. One is painted in reddish tones, and the other in bluish tones. Now, you will make a choice as to which room you will take, however, you will base that decision on which color you prefer. You'll either choose the red room because you prefer red, or you'll choose the blue room because you prefer blue. But it boils down to the preference, not the choice. The choice is based on the preference. Are you following me?
A man has, standing before him, a gorgeous woman and a handsome man. He can choose which one to date. He can either choose to date the one he's not attracted to at all, or he can choose to date the one to whom he is attracted. So, which one would you choose? I think you, like me, would choose the one to which you're attracted. In this case, that's what he'll do, and he'll choose the handsome man. The woman, as gorgeous as she is, just doesn't make him feel the same way that the man does.
So, yes, choices are being made, but people aren't choosing to be gay, lesbian, bisexual, or trans. They're choosing to live happy lives by not trying to suppress their desires just to make other people, who are made uncomfortable by their desires, happy.