You might interested to read the account of monogamish woman who hires sex workers.
The idea that marriage about safety and security and once you have those basic needs met you can get other needs met. That's why the 'no to sharing' at the party and no to twins 'until we've been together' makes sense. SH needs to feel secure in the relationship, to feel like she is first in Tav's heart and *then* involving others sounds fun
Drows is the different scenario though, because party doesn't end in sex or anything. Drows are just sex workers and she does it with Tav obviously. She is just kinky and ok with some casual sex and ok if Tav wants some, just a reminder that she doesn't ask or do herself, she is fine with only Tav, she joins drows if asked. Thats why I consider her in "open monogamy" with Tav. And yes she basically proposes marriage at her lower city confession.
If they fix Halsin interactions - everything would fit.
