Here come the 2 cents from someone who shouldn't offer ANY cents...but...
Your post worries me. We're about to step into the warm waters of a poly relationship and are still working out all the kinks. Our situation is much different...BUT...we've been looking for 13 yrs and have been married for 10. We would WANT HB to be a part of EVERYTHING - the family, the house, the mortgage (and the deed) - hell...health care coverage if we could swing that! HB is stuck in the "play" mentality - and is worried about overstepping boundaries and intruding on our "real life" and all. I'm wondering if this is a fantasy come true...and reality isn't as...well...fantastical for your lady friend.
The marriage seems to be like a slap to your face, honestly. Is there a reason for them getting married? Was there a financial commitment to the wedding prior to the onset of your relationship with them that they can't afford to back out of?
I guess I'd feel more at ease if you were being included in decisions, plans AND the wedding (as a bridesmaid perhaps?) and that following that ceremony there could be a ceremony held for all of you. In my stratosphere, we have collaring ceremonies instead of "weddings" but when undertaken with the right air of solemnity, it is just as binding.
Again..not even sure I'd listen to me at this point - but that whole post just gave me a weird vibe.