Oh I see.. It makes sense now. I remember watching "Le Nozze di Figaro" and didn't really know the reasons behind making a women playing the young man character.

I'm quite novice when it comes to opera but I remember watching that one and was surprised that I was halfway into it without blinking.

If I remember correctly the women played a young man and then during the story she plays a women as a "disguise" then later reappeared as a man.

I'll check out "Fidelio"