Why would a guy who has a girlfriend be jealous of another woman talking to a guy?

Venus-Aphroditex

This guy cheated with me a couple of years ago. We had sex once while he's been in a relationship. (it's bad I know) I usually see him where he works because he works at a store that I go to. A couple of months ago when he saw me talking to another guy (his coworker) he came over to where we were and interrupted our conversation. He overheard this guy name Rick (his coworker) telling me that he had done something fun. And then he came over and stood between us saying (how was it?) I don't know why he would do this.

Today when I saw him I told him I got a job working at a museum. And I told him I wasn't sure about it because it's really classy and elegant. And then he said I'm classy and elegant.

Why would a guy who has a girlfriend be jealous of another woman talking to a guy?
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