I suppose.

That said, not all two-person relationships are the same type of relationship either. My relationship with my husband is not very much like my mom's (former) relationship with hers, despite both relationships being monogamous... indeed, it has more in common with several of my friends' poly relationships.

That said, though, we can certainly ask whether there are more ways for N people to be in relationship at a time than for 2 people to be in relationship at a time? Yeah, I would expect so, I guess. (The prospect of itemizing them, as you seem to be trying to do here, seems both daunting and not terribly useful, though perhaps entertaining.)

Does the difference actually matter that much, in terms of what leaves people better off? Maybe. I'm not really sure. I'm not even sure how to approach the question.

Certainly, the more willing people are to explore a wide range of relationship-space, and the more able they are to recognize what leaves them better off in a relationship, the more likely they are to find a way of being in relationship that leaves them better off. But this seems no more (though no less) true for being willing to experiment with polyamory as for being willing to experiment with their nonpreferred gender as for willing to experiment with various sexual kinks as for many other things.