Why does my friends with benefits gets jealous?

Anonymous
I used to date a guy, but he was too young and wasn't ready for a committed relationship, so he broke up with me and after a while, we became friends with benefits. At the beginning he wasn't jealous at all, he even said that he was ok with me meeting other guys. But now something has changed. He is way more jealous than when we were together. When i casually talk about other guys in a very innocent way, most of the times he gets jealous, and sometimes he even insults them. Furthemore, I told him that an older collegue of mine (which is ugly, so not a threat for him) likes me and even text me often, he went on and said: "tell him that he should not desire someone' else woman".
I am so confused by this behaviour, i dont know what to think.
Why does my friends with benefits gets jealous?
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