Why is he jealous if we are just friends with benefits?

My best friend, whom I am also best friends with gets jealous/annoyed when I hang out with this guy that I am also talking to. When I tell him I'm hanging out with this other guy, he threatens to go be with another girl. Also he gets very sad and is not himself when I don't answer the phone or his texts right away and starts to worry or assume the worst. What does this mean?
Why is he jealous if we are just friends with benefits?
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