why does my friend with benefits gets jealous?

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 disappointed. 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.

Ask him if he wants to move your friends with benefits situation into a dating situation (that's if you want that too)

He wants you to be more than FWB or it seems like he already thinks you are more. It is time to have a talk and either become exclusive or move on. Jealousy and more than friends feelings have no place in successful FWB relationship.