To those of you lefties complaining--get a life! The show is meant to be entertainment, not leftist propaganda.
Moreover, given the current P.C. climate, I can predict exactly where this would go if there were a female Dr. Who. We would have episodes focused on the "historic wrongs" perpetrated against women. Dr. Who would be on a mission to push feminism in Victorian England or some other less inspired time. (Yawn). Since Dr. Who is married and has had a few trysts with women, we would suddenly have episodes obsessed with the new Dr. Who exploring her sexuality and feelings toward men and women. (Yawn). And what was a mildly eccentric, humorous, adventure program would fall into the same PC trap that has killed many another show, and end up being cancelled.
However, here are my thoughts on this conundrum. Time Lords, like Niven's Puppeteers, actually have three sexes--two non-breeder sexes that each contribute some genetic material and a third, breeder sex, which is unintelligent, but carries the baby Time Lord. Obviously, Dr. Who can't transform into an unintelligent breeder, so he must alternate between the two other sexes, which always appear (to lowly humans, anyway) to be white, male, and British.