fwb asked if I was ganna have sex with someone else

we work together, just made out for a bit after work and I told him I had to go, that I had plans but we could meet up later if he wanted to. I guess he assumed I was going to see another guy and said "are you going to fuck him? I don't want to if you're ganna be with someone else" And I was like "what? I wouldn't have sex with you and anyone else in the same night anyway. sounds like too much work for me" and he was like "i have to take your word for it I guess, I've already decided I'll never know if you do me after anyone else anyway, you'd probably never tell me" I also got a call from my friends brother and he saw my phone and was like "who's evan?" And I went outside to take the call, came back in and I said he was my friends brother and he goes "what? you're banging your friends brother too?" i saod no, her phones broken i had to call him to talk to her and he was all weird after that. what the heck is all this about??