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
Apparently she's willing to let you sleep with Halsin before you've slept with her though. So I'm not sure you're argument holds up when he's involved. She'll refuse the twins saying she wants you first but has no issues with letting Halsin have you first. It's weird.
