The bad thing about this type of genre is that if there isn't a limitation, you're basically a god, so why not just have one wowman and change her every single time you have sex, that makes the story boring and repetitive to a degree.
Another issue in terms of story concept, would be the tremendous energy required to perform the transformation. Otherwise, the story is simply unlimited magic the author can call upon at any moment to get out of any given situation (i.e. you'll lose most readers quickly over a lack of any understandable rules of the story universe).
If you have to account for where the power to transform people comes from, you'd limit the characters to somewhat 'human' restrictions (which I suspect the Swarm Cycle universe does by using aliens for the feat).
Generally, the idea for any story is to lay out the universe in the first several chapters, where most readers are willing to allowing authors free-reign to invent outlandish situations, but after that, readers will criticize authors for not adhering to their own rules, or for randomly inventing new details as the story unfolds.
It's one thing giving a young kid unlimited powers to do whatever they want, but it's another entirely to restrict how and when he can apply it, making it a challenge how he manages such powers.
Other than that, the idea sounds like the 'feminization panty' stories, where putting on a magic pair of panties transforms guys into women (a common theme on ASSTR and other sites).