Woman self-defense training can be a tricky task for any good male (and even female) instructor to to handle. In our modern world "political correctness" requires us to follow gender equality principals in our public and professional life to its utmost. And this creates significant hurdles in setting productive self-defense training environment for female students.
The gender equality principal requires us to treat male and female students as "equal" which often (incorrectly) implies "the same", when right (but hard to balance) approach is "equal, but different".
I won't go into accent roots of gender specialization in human society. What is important for this discussion is that body-build, physical strengths, weaknesses, and limits are different between genders. And so also social risks and patterns of violence.
In male vs. male self-defense training the situation for both partners is about to be equal. On the other hand, in male vs. female it's anything, but equal. When female on female and, even, female on male hostility undeniably exists, those are relatively marginal cases and, even more important for our discussion, they present lesser physical challenge then more common "mainstream" male on female violence.
Sure, handling aggression by physically weak female or under-age hostiles should be addressed in good self-defense class, but it's mostly done by discussing how not to cross a gray line of necessary and proportional response, that can turn you from victim of aggression into a guilty party and put you into legal trouble.
Self-defense training, in general, should concentrate on handling of most common and most difficult scenarios. From this perspective, in male vs. female pair training, male partner generally "wasting" his time. This should be well understood and properly managed in good mix-gender self-defense class.
In woman-only self-defense classes it's absolutely critical to have a few male assistants who can provide female students with challenge of threatening physically powerful "hostility". Otherwise your students may get confidence not backed by abilities and realistic self-assessment (and this is dangerously insecure combination).
And for necessity of woman self-defense training... I think it's absolutely indispensable. Even I'll be here for my wife as long as I can, my daughter will not let me to be her "bodyguard" even on her first date (well, when I have a few more years to teach her, so far she's not very interested, to my dismay :), and dating, unfortunately, is a well known high-risk scenario...
And for man and woman equality... As Jack London said in his "Smoke and Little" novel: "They shared load equally - each picked as much as he can care" (quoting from my childhood memory and I like the idea even I may owe to memory of Jack London for quote inaccuracy :).