Every answer to this question is inherently speculative (although an official answer may well come with the new series), but my assumption has always been that changing sex is less likely.
That is, Time Lords can regenerate into male or female forms, but changing sex involves a significantly greater physiological change so requires more regeneration energy and is thus less likely. Alternatively, it may be possible only when sufficient damage has occurred to the relevant organs.
If this is true, we would expect 13 to be followed by a string of female Doctors (but a male 14 is also possible).
If there is, say, a 10% chance of changing sex at each regeneration (and excluding the metacrisis, but not the War Doctor), a string of 14 male incarnations is the single most likely outcome, with a 23% chance of occurring.
Changing at some chosen time, resulting in N male Doctors followed by 14 - N females, has a probability of only 3%; this includes the position in which we find ourselves now. But there are 13 possible values of N, resulting in a "single change" probability of 33%; this value is more informative.
The probability of there having been more than one change by now is thus 56%.
This implies that a sex change is somewhat overdue, but it is by no means unreasonable that one would not have occurred yet. The probability can, of course, be tweaked to fit reality even better (or according to the writers' whims); a lower probability will make the model fit better.