People don't choose to be gay/lesbians or trans, and there's tonnes of evidence in support of this.

Sure. But if I, a straight man, slept with a man, then I would become a homosexual by definition. Or if I pursued becoming trans. Now it's unlikely I'd do this but theoretically it is part of the set of possible actions..

There is no action a white man or woman can take, not surgery they can pursue, that will make them brown.