I have noticed that most often, in these movies, there is some lure, or twist of fate, or coercion, or otherwise inescapable circumstance involved. Going back in time, this theme becomes more and more pronounced. For example there was a comedy that I think came out in the 90s about two undercover cops, who as part of an assignment to infiltrate a school I think, were required to pose as females. Tootsie, Mrs. Doubtfire, etc. is the same deal. There are a million examples. It's like saying, I'm not really "that way", I'm just a guy who was in the wrong place at the wrong time and got swept up in it. It makes the story easier to digest for those who are intrigued by the concept, but who still view it as an aberration. So the choice is taken away and replaced with some higher purpose or motivation, other than simply wanting to.