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I think it sort of depends how the breakup was. If you just didn't get along and mutually broke up (or in many cases it SHOULD have been mutual, only one person is too blind to see it at the time), then it's nobody's fault and no reason to act all weird. If the X was cheating, abusive, or otherwise a bastard, than friends have every right to hate him (or her), and really, the bastard deserves whatever shit he (or she) gets.
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I think your directions to your friends is a mature response, AND leaves teh door open for a possible reconciliation one day, which ISN"T necessarily a bad thing, but Usually is....
Your friends aren't necessarily immature, they're trying to compensate for any post-break-up weakness you might have by being strong representatives for you. I'ts natural for close friends to want to protect their loved ones from those who have hurt them. Some people DO get kicks out of making it about themselves also, but they have your best interest at heart.
I still think "cordial indifference" is a better way to go than friendliness, however...