He told me not to be whoring around.

I went to leave my boyfriends house to do some shopping and he says “don’t be whoring around” in a joking way. Now we fully trust each other . I’ve never done or said anything for this to even be okay!!!! I asked him why he would say that and he smiles and says he’s just joking. I thought it was pretty mean.....why would he say that to me????