Perhaps the problem is more insidious than just being a matter of the gender of the instructor; it's their own bias.
One time I was at a large seminar taught by a number of shihan. As it happens, there was one female teaching. The quality of all the teachers was, of course, excellent, and it was an great event. But... here is my observation. Almost half the people training on the mat (well over 200 in all, I think -- it was a large, international event) were women and yet, when picking uke out from the keen and willing participants, none of the first four instructors, including the woman, chose a single female uke.
Now, I am of the (mainstream?) opinion that gender really doesn't matter -- in fact it's one of the positive qualities of aikido -- and let's just get on with training (actually that's not quite true; I have found that dojo with women instructors are less likely to suffer from silly machismo issues, but hey). But on this occasion I was uncomfortable because statistically, picking uke time and again from an almost 50/50 pool, this was not a random result. I wasn't there gender-counting; but I became aware of this happening because there were women in our group who couldn't fail to notice. So when I see a thread like this one on aikiweb, asking what gives with the low number of women instructors, I sometimes think the pattern is more entrenched at the top than we would like to believe.
I was pleased that on this occasion it was my instructor who broke the pattern; not conciously, I'm sure -- he was just teaching aikido -- but simply by picking uke, and not applying a (subconcious?) bias.