I've been following your thread with interest. Your question is one I struggled with and turned over in my head lots at the start of my relationship with my partner. He has had poly or open relationships for much of the last 15 years or so.
For the time being at least, I have come to the conclusion that while in theory, poly relationships have no difference in a moral sense from mono ones, in practice things aren't so simple. In practice I see secondary partners be hidden as partners from the outside world because of real worries about loss of family or loss of livliehood. I see constraints be placed on relationships between individuals because the partner of one of them is struggling - from real compassion for the person who is struggling. I see mono partners work to deal with having a partner who isn't always able to support them when they need it. I see people who are one half of a married couple deal with significant amounts of hurt to be able to deal with their spouse's new partner.
I theory, poly is just like mono only with more relationships. In practice, it happens in a world that doesn't accept it and from what I have seen, that situation makes it more likely to cause hurt.
My conclusion so far has been that if I were to find myself again with no partner that I may explore having multiple relationships in the form of friends who I sleep with but if I have somebody in my life who I would refer to as my partner and who I feel okay with others thinking of as my partner, then I want that relationship to be a mono one for both of us.
This doesn't mean that if my partner came to me and said that he wants very much to be in an open relationship again that I would dump him and not speak to him again. Things would have to change between us for sure but he is one of my closest friends and that doesn't need to change even if we decide at some point in the future to no longer be partners.
I think I am super cautious about this stuff, though. I have never dated in my life - partly because of worries about hurting people who may become more invested in a relationship than I want to be. I've also never had a really nasty break up. My romantic relationships have always ended with respect and kindness and on friendly terms so I don't see that relationships always end (other than through death) as a truth. They change, for sure, but I don't have experience of the end of a romantic relationship meaning the end of the relationship altogether.
I know that I have a tendency to be overly cautious, though so very much understand that others make different decisions.