I have a theory about female wizards on the Discworld, which is just about possible to defend on the basis of the first three books (The Colour of Magic, The Light Fantastic, and Equal Rites), but which also owes something to much later ones (The Last Continent and Unseen Academicals).
Unseen University is the Discworld’s premier college of magic (in its own mind, at least); not the only one. We already know there are wizards on Krull, which is almost completely separate from the rest of the Disc. We know that at least one of them (Marchessa) was female, and that this was not remarked on.
Buggarup University in XXX is another example of a separate college of magic, though it too is all-male.
In Unseen Academicals, we see that in fact Unseen University has rivals not only in places separated from the rest of the Disc, but also on the main supercontinent. The UU Dean has just set one up in Quirm; there’s a visiting professor from one in Genua; and I recall hints that there are others, with long-standing rivalries between them. (And Ponder Stibbons can play academic politics as well as anyone, thank you very much!)
We also see that there are differences in academic culture. UU wizards must be celibate (there’s a hint in The Colour of Magic (and again in Sourcery) that it’s only straight sex which is outlawed, but the possibilities thereby left open are never explored. This does not appear to be the case in Genua. When Ridcully heard that a Genuan wizard had been “named in divorce proceedings”, he just assumed that the Genuans didn’t prohibit their wizards having sex with women. He had to be explicitly told that in this case it was gay sex.
So UU has a “straight sex explicitly outlawed” and “gay sex doesn’t happen, does it?” culture, but accepts that other universities have different cultures. The one in Krull even accepts women, and did even before Esk’s time. Maybe others do too? Maybe, in fact, UU is the sole remaining holdout?