Well you're book is wrong then. No where in the established canon of the show does it state a Timelord cannot change sex.
If you want to be sexist about it, that's fine. But stop trying to pretend you are not.
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.