Interesting comment Marion. There is actually an "answer" song to "A Boy Named Sue" by the same author (Shel Silverstein). I think it is actually funnier than the original, as it tells the same story from the father's point of view. I have a song which is sung from a woman's point of view called Johnny Petto. I often wistfully think that if I ever hear it sung well by a woman, my own efforts will become redundant. It would be so much easier for an audience to believe that a woman really was the character.
There are, no doubt, plenty of men who can pull off this sort of thing though. Bert Jansch's excellent "Rosemary Lane" is a case in point. I would not want to make a sweeping statement about whether it always comes off or not though. Best to judge on a case by case basis. I can't think of any real folk song which I would want to change the sex of though.