Was MaryBeth a MAN??

I hope this is not a dumb question, I missed the first part of the movie. Was MaryBeth, the black beautiful woman really a man?? I know in real life she is a woman but its really hard to believe her being a man, she is far too pretty and feminine, it was not believable, she played the role well of course but I don't think anyone really bought the idea she was a man. Was this how it was? Benny seemed so "manly-man" its hard to believe he would live with a man, I guess towards the end he realized it.