Yeah jes, that comment about being a slut made me laugh.
This could be one of those age/experience things, but at my stage of life if I were in a similar situation and the man called me a slut, I'd respond with "Then what does that make you?"
In any event Resilientgurl, if I were you I'd come to terms with the reality that in this case an apology would be complete bullshit, so just move on.
When he called you a slut, his emotions were high because his dick was hard and he realized he wasn't going to get any. Plus apparantly he wasn't accepting the FWB thing as much as you were. If you want to try to force an apology out of him, when you know damn well it'll only be words, go for it. However, realize it's only you that will be putting forth any energy, and negative energy at that.
Don't push the river. Life's too short to fret about this type of thing.