Granted, the Mercy and William situation is complication, since she's currently still married to a man she can't stand, but has to pretend, and, up until recently. was sleeping with both of the Stone siblings and the Raven (who turns out to be both of the Stone siblings.) I agree that the eventual relationship will like be monogamous between Will and Mercy.
And the Peter/Marta situation is incredibly complex, because is it still monogamy if if time displaced versions of the couple join in occasionally? What about alternate reality versions? But I agree that the poly-ship of Peter, Marta, and Sebelah is a possibility (and certainly canon in some realities).
Monogamous
Astrid and Ellistair- This is partially built out of political expectations and scrutiny the couple faces.
Open Monogamous
Yusef and Aurelia
Gwynn and Kaarim
Open Polyamourous
Then we have Twelfth Night. So, Olivia is married to Sebastian, but they have a mostly loveless (but friendly, sort of) marriage that is sexual to conceive children. By arrangement, both are allowed to take a lover so long as no children are conceived. So Sebastian has Annette (I think?) and Olivia has Viola. Viola is expected to marry and have children, so eventually she would have married Bastien, bisexual man who prefers other men. They would have been each other's beards, so she would continue her relationship with Olivia, while he could take male lovers on the side. He and Viola would had a sexual relationship, with Olivia involved at times. I think Shakespeare would have been proud.