Another term to explore at some point -- commitment. What does commitment mean to him and to you? Why *wouldn't* you make a commitment, nominal or otherwise, to someone even if you were then going to bone other people, perhaps a bunch of them? After all, that's what people in open relationships and swingers do, and it seems to work for many of them.
Is "commitment" exclusivity, focus, promises kept, a shared future? Only the first definition would preclude sex with others, though the second definition could arguably be somewhat compromised by sexual non-monogamy depending on how you conducted yourself.
Though of course, as others have pointed out, poly is not primarily, or sometimes at all, about sex.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.