The struggle continues...

Matthew Yglesias got a letter with a presumably unintentional bit of sexism. Some of his commenters have the balls to try to justify it. Funny stuff.
But, on the related note of how people just don't seem to get it, after two years of living in sin I have occasionally had the not-quite-pleasure of being addressed as Mrs. Mouse'sboyfriend'slastname. I never know what to say. Sometimes I can tell that just simply correcting the person, "Ms Marcotte, thank you" works. But sometimes you know that's opening up a whole can of "Are you married?" worms that I don't want to get into.
What to do?