I have a guy I work with, and he used to make snide comments about alcoholics around me, not knowing I was one. I can really empathize with the fist clinching reaction some have had, since I had that on occassion myself with this one.
So one day while he was yukking it up, I asked him straight in the eye "What is your grips with alkies? Think very, very hard before you answer that, because I want to know what your problem with my brother is." For dramatic effect, I grabbed a pen and paper and told him to start slowly so I can get it all down.
So I skirted the issue of having to tell everyone in the office about my affairs and made him feel pretty small in front of my coworkers at the same time. We still get along fine, and even though some may disagree with the methods, I feel in my own way he learned a lesson he should have learned long ago. He doesn't even joke about gay people anymore (good thing, since my brother really IS gay, and I'd hate to do it to him again... he's really not a bad guy, just says bad things).