Why is my ex friends with benefits jealous of my friendship with another guy that we work with?

So me & my ex friends with benefits has a bad breakup in which didn’t end on good terms.
Lately he keeps teasing my guy friend ( our co worker) about me & him being friends now instead of just asking me.
Why do he even care whom I’m friends with & whom I’m dating if we’re supposed to be moving on?
Why is my ex friends with benefits jealous of my friendship with another guy that we work with?
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