L'incoronazione di Poppea
When listening to this opera it is important to remember that in the 17th century men often sang women’s parts and women sang men’s parts.  A part such as Nero would have been sung by a castrato.  Nowadays it can either be sung by a countertenor (a man) or a mezzo-soprano (a woman).  There are scenes in this opera where the actors disguise themselves as the opposite sex.  In the 17th century opera audiences did not find anything unusual about a man pretending to be a woman pretending to be a man or the other way round.