Why my friends with benefits gets jealous when I innocently talk about other guys?

I have this friends with benefits relationship with a guy that I use to date. He's very young and wasn't ready for a committed relationship, so after a while we became friends with benefits. At the beginning he told me that I was free to meet other guys, but now something has changed. When I talk about other guys in a very innocent way, he is jealous and sometimes heeven insults them. Furthemore, when I told him that an older collegue of mine (which is a very ugly man, so it's not a threat for him) likes me and even send me messages, my friends with benefits told me: “tell him that he should not desire someone’s else woman, even the bible says that”.
I really don't understand, everything is just so confusing
Updates:
+1 y
He was the one who wanted to start this friends with benefits thing, so I don't understand this kind of behaviour
Why my friends with benefits gets jealous when I innocently talk about other guys?
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