^Yeah, I agree with that assessment. Perhaps it's as simple as different Timelords having different gene combinations. It could be like a Punnett square, with Male regeneration being the "dominant" trait. The Doctor is MM, people like the Corsair are Mm, and some Gallifreyans are only ever female and are mm. Some of them will always be male. Some will always be female. But there are those out there that have a 50/50 chance every time they regenerate of being either male or female.
It's also implied that most Timelords actually have more say over who/what they turn into than the Doctor does.
Well, barring a black of a woman Doctor (either of which - or both, a black women - I'd like to see)...I was recently watching the British film "Pirate radio" aka "The Boat That Rocked", and I really thought that Tom Sturridge would make a cool Doctor. [yt]http://www.youtube.com/watch?v=K6-QfQ1OJqQ[/yt] Though he may be a bit too much like the current Matt Smith (and who wants just more of the same) - and he may be a bit too young and "prety boy" for some people's tases. But I could really see him in the role.
I don't know that it's really "sexist." (At least, not in the negative way that word is usually used.) It's simply a matter of what one thinks would or wouldn't be appropriate for the character. Gender often conveys a heck of a lot about personality. So, depending on your point of view, changing the gender might change the character TOO radically.
Nothing says that re-assignment surgery couldn't be had outside a regen-cycle--or the cycle modified?
I'm not sure Doctor Who is quite the right medium for exploring a realistic portrayal of trans-gender issues.
Well, wasn't there a sci-fi story where a man travelled in time enough to become his own mother and father--a closed loop? I wonder what if anything will be done with the Doctor's daughter...