Compared to some of the other posters, my experience is limited. But IMHO, there are two issues. Why teach women to use guns at all? That has been well answered above. The other issue is why teach "women ONLY" classes?
My answer to the latter is that any time you try to teach men anything about guns (military excepted since the students are under rigid discipline), they will second-guess you, try to show off their "knowledge", interrupt with old war stories from TV and magazines, insist on bragging about some hunting trip some family member took twenty years ago, spout garbage from last month's G&A, and in general try to be macho show-off know-it-alls.
And in mixed classes, it doesn't get better, it gets worse because the men have a built-in female audience for their muscle flexing, chest puffing, and bragging. And if the instructor is female, she had better be able to keep discipline and prove she knows as much or more than the class he-men.
Women, on the other hand will admit they know little or nothing about firearms and generally want to learn. They will take instruction and absorb information without trying to trip up the instructor or prove they know more than he does.
One thing the male instructor with an all or part female class has to be careful of is preening and sexual innuendo. "Coming on" to a female class member is a no-no, even if there is no legal or moral barrier and there are indications an advance would not be unwelcome. Don't do it. It is also a good idea for the instructior to have a female in attendance who will be "on his side" if there are quesitons, like a wife or a female police officer.