I am so sorry you are hurting, but don't let these 2 experiences leave a bad taste in your mouth.
I don't think this is about "monoamory" or "polyamory." Or about "monoships" or "polyships."
This is about honesty and truthiness and interpersonal relationship skills and willingness to just spit it out
to your people so whatever it is can be dealt with openly and in constructive ways. Rather that destructive.
That's why my playbook
exists actually written out. Potential is invited to add their own. If all agree to fly under that banner and try dating? Alright then. Honor the colors you fly under.
Don't think you can hack those rights and responsibilities? Just don't play here then. We could be friends and leave it there.
Maybe next time go a whole lot slower? Hold off on sex. NRE brain cascade is bad enough without sex hormone cascade in the cocktail. The brain is a power organ.
And in going slower, you could give yourselves time to pick up on clues on how this new person's personal relationship skills set is really like OVER TIME and in different environments. At home with their people? Your people? In public? At work?
Not just out the starting gate when everyone is in their "best dating behavior mode" and it is hard to tell true colors yet.