I see no problem in exchanging harmless humor between two friends in the context of a long history of friendship and understanding that no party has any ill will towards each other or the outside world.

I don't believe it's any of your business what I talk about with my friends in the privacy of my home. In fact, I find your attitude ignorant, dangerous and a little scary.

I'm not saying the Rapgenius guy's joke was any funny or appropriate it at all. It honestly didn't seem that funny to me. But I don't think "making bad jokes in private" and "being a good person" correlate very well. There are other things I prioritize, like doing actual good.