Most drag queens (as opposed to the transgendered) do it for entertainment or as a profession. Very very few actually wish to be women and are quite happy with their bits and pieces. Many, if not most are actually straight.
Although the terms are jumbled in popular culture, I would say a drag queen is gay or bisexual (because of the 'queen'), otherwise they'd be a transvestite or simply a cross-dresser. But you're right, most cross-dressers are not homosexuals. I'm afraid most people can't or won't wrap their heads around that, though.