I'm a little curious. Is there a reason the issue of additional relationships, in whatever form, cannot be "postponed" for a little while? Do these decisions have to be made right now?
A lot of people introduce polyamory as a concept, let it rest for awhile, and then continue the discussion later, if needed. Or if this is a deal-breaker, decide whether to stay together or not.
I think Newguy already made his position clear about additional relationships: no. If PK still needs polyamory, she needs a different fiance. If she needs Newguy, she needs to give up non-monogamy.
My 2 cents.