It depends on what kind of repertoire the two of you have. If he's saying it with a smile and for some reason believes you like being spoken to that way (some women really DO like being told that they are trashy slutty whores), then it's really nothing to worry about.
First, tell him that you really don't like it, even if he's joking around. If he continues to call you derogatory things, then there's clearly some level of aggression on his part. Maybe he's jealous that you are in fact seeing other guys (yes, even if it's agreed that you are FWB and it's supposed to be okay to have other partners). This is definitely not a good situation. Means he has little regard for you feelings since you've asked him to stop and it just continues. It could mean that he's possessive and jealous. Could mean that he's just mean and enjoys subtly hurting your feelings. Any of those scenarios is strong indicator that it's time to dump him and you totally should because the behavior will almost certainly continue to grow and could morph into other things, eventually posing a risk to your safety.