Originally Posted by dron
I know I need the negotiation to be happy, but how do you firmly communicate that without an implicit threat?
There is always an implicit threat in any negotiation of boundaries--and that's a good thing. For a relationship to be healthy, it has to work for each involved, and each of them has to have enough self-respect to be able to walk away if it's not working and the other person(s) aren't willing to help it work.
So, implicit in any negotiation of boundaries is the understanding that if an accord can't be reached, the relationship won't work and it's essentially over (in that form, at least). Poly, mono, platonic--it's the same for any of them.
If she's not willing to help make it work, then you're best served walking away until she is willing or you find somebody else who is willing to help it work for each of you. That's not only best for you, it's best for her, as she'd not be able to make the sort of commitment necessary for a deeper relationship.