I have to disagree there. Although I would personally be more liberal with such decisions I believe it's the author's right to say that the gender and/or race of the character is a part of the text and that the roles must be cast according to gender, race and other specifications he/she considers to be essential. A standard Dramatists Guild contract gives the author veto power over casting. (That's how Arthur Laurents was able to prevent Patti LuPone from doing Gypsy until he changed his mind.) And if there's any fuzzy area I side with the decision of the author because I support his/her right to determine what is and isn't an essential part of the play.