Vera, nolo, altagirl --- great points.
This topic came up on the Fear thread, too, and I'm afraid I'm just repeating what I said there: I think there's a role for male instructors in all-women groups, but that it's important that the participants be made aware that the instructor could be male, so that they get a choice.
Personally, I've only ever been in mixed groups, except for one day which by accident turned out to be all-women learners with a male instructor. I found it a great boon that the other participants were like me in terms of ability, age and learning goals, but it didn't matter to me whether they were women or men. Similarly, if an instructor can improve my skiing I don't care whether it's a man or a woman.
Having said that: (1) based on a small sample (5 or so different instructors/classes), women tend to have more similar learning goals to mine (very technique-focused, as opposed to trying to ski harder slopes); (2) one woman dropped out of the ski class I described above specifically because she was tired of the stupid sex jokes that the male ski instructor would crack on the lifts.
So sometimes gender can factor into things --- it depends a lot on the person. I agree with Vera that we can't make generalizations. On the other hand, I feel for Rusty Guy's predicament. I think it should be up to the women skiing to choose whether they want a woman or a man to teach them, but if they're signing up for an all-women's group, there's a better chance than average that it will actually matter what the sex of the instructor is.
Meanwhile, a helpful epicski person pointed me to some great-looking ski clinics, either co-ed or women's, taught by a couple of world class female skiers, at Ski Escape. Great!