Why does he want to share me?

My boyfriend and I are not married. But we have been together for two years.

He used to be in poly relationships but now, we are monogamous.

He has been mentioning a lot lately about a friend of his having a crush on me and how he wouldn't mind having us sleep together.

First, I don't really know the man. Second, my fear is that one day, he will come to me wanting to sleep with another woman and tell me it is unfair if I get to do it too.

I have already been in an open and poly relationship. But he keeps arguing that it wasn't done right. Either way, I tried it and I was not secure at all being in that kind of dynamic

I don't know what to do other than talk to him. I know he loves me. But at the same time, despite our time together, I still have a fear that he will leave me for some other woman. And I don't know how to communicate that to him.
Why does he want to share me?
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