I know it was meant as a joke, but I (and likely only I) didn't find it funny, and found it rather insensitive at that. You should have stopped at "I'm sorry."
Copping an attitude about it doesn't help, either.
And yes, I know you're very much the "no PC for me" and "the more insulting the better so I can knock you PC lovers on your butt HA HA HA" kinda guy, but when you do actually offend someone, regardless of whether you think it's legitimate or not, sometimes you just gotta take the lumps, even from your friends.
Food for thought.