Been there, done that.
It never ends. There are only so many permutations of this fantasy, and they all end up with the male protagonist enthusiastically and irrevocably abandoning his manhood.
It always gets to me.
I have a couple of quick ideas:
My chart of possible reactions. The experiment starts off each man with a choice of two incredibly sexy feminine outfits. Eventually, as the experiment progresses, one of the choices becomes less and less sexy. When the man is finally presented with something masculine vs. something feminine, he chooses the feminine.
Second, a story about how a man comes to discover that he loves women because he wants to be one, and he begins discovering his feminine side.