Kagan's mistake here wasn't that it was offensive. I don't think there was a rational reason for anyone besides his ex-girlfriends to get offended about this a

Kagan's mistake here wasn't that it was offensive. I don't think there was a rational reason for anyone besides his ex-girlfriends to get offended about this and there's no point in bothering with people who look for reasons to be offended. His mistake was that this made him look petty and childish ("Oh look at me, I'm in front of people so now I can call people I'm mad at names!") while serving no positive purpose.