Original poster here.
Thanks to you all for your replies.
About leaving the dojo - I should stress that overall it is a wonderful place. The dojo is technically astounding, very martial, challenging but supportive, offers many many classes, and is full of good students and good teachers. I have plenty of great training partners there (juniors, peers, and seniors alike), and a number of teachers I have great rapport with. (Mary Kaye - the other instructors range from shodan to rokudan, some are (3rd - 5th dan) women, and most are excellent. Technically great, of course, but also sincerely dedicated to teaching, and impartial about whom they help out. Btw, some of them have noticed the frat boy problem and tried to do something about it, eg by having people switch partners throughout class rather than staying with the same person the whole time, but without much effect.)
In fact, just reviewing all the good things about it helps put the problem into perspective. There are maybe 150 or 200 members, ranging from no-kyu to rokudan, with every rank well represented, and most of them not snobbish about who they train with --- and the "fraternity" comprises about 8 or maybe 12 jackasses.
Jeffrey, I'd been telling myself the same things you wrote. But I did need to hear it from someone else.
And thanks, Rachel, for the words of wisdom. Again, it helps a lot to have someone tell it to me.