I like Kathryn's mention of "guys/girls night out". Those nights can be very fun and enjoyable, where men don't need to worry about how women perceive them, and vice-versa.
Everyone seems to want to change Mr. Linden's mind here -- which likely won't happen. This is an interesting point of view, so much so to some of us it seems "alien".
There are some issues here, hidden deep within words that will never come to light publicly for fear of retribution. Favoritism or segregation between groups is usually because of some kind of past experience(s) with those groups, or, because of environment or upbringing.
This is aikido we're talking about, not a poker group or treehouse club. The way I see it, it's only "right" or "wrong" if it somehow violates how the universe works. And I can't believe that practicing with only the people you like is a true representation of what O'Sensei tried to tell us.
More comments? I love a good, non-PC thread.