Well, to be fair, at gatherings where you don't know everyone, there is often a tendency to pick as Uke's the people who look like they're probably able to do ukemi well. So your eye looks around for the male victim... er, volunteer with obvious athleticity, etc,... not out of any subconscious desire to avoid women or the smaller, weaker males (remember, they're getting left out disproportionately, too), but out of a desire to pick someone out of a group of unknowns who can do ukemi and do it without getting injured. I.e., avoiding injury while making a clean teaching point is more on most peoples' minds, I suspect, than even considering gender equality and other side issues
While I realize that some people might like to see big guys getting thrown around, I don't think picking men based on ukemi ability has any logic. In my experience, women often have the most connected, smooth and all around great ukemi skills. Big, musclular guys often seem to be rather stiff and heavy and less likely to connect well to the technique. If the point is to look tough as nage to an audience of non-aikidoka, then perhaps this choice would make sense, but at a seminar choosing men because of their assumed ability makes absolutly no sense.
My two cents to the general picture: I agree with the folks that say the best way to get more women involved is for women to decide to be involved. I don't want to be chosen for special priveleges or invitations based on my gender but I am inspired by strong female presences in the aikido world. As I gain more influcene in my aikido community, I plan to encourage and hopefully inspire more women to participate, but equally so for men. I think we should acknowlege the desire to see more women on the mat and teaching and then we should all just get out there and train with each other, regardless of gender, race or anything else.