Only complicated if....
Really - your situation is not (in theory) that complicated at all - or uncommon.
But as someone already mentioned I think - and something ALL of us rave about constantly here - communication is in order here. It doesn't sound like the 3 (potentially 4 ?) of you are all sitting down at the same table and talking OPENLY & HONESTLY with each other yet.
Until EVERYONE knows what everyone else is thinking and what their priorities are, there's huge risks in doing or ASSUMING anything !
RE: your comment about "Other issues at play"......
"Marriage" has been undergoing a pretty significant evolution for some time now and it means different things to different people. One of those things that I think more people are willing to open up about than in the past is the concept that it may be more of a "legal contract" - like a business contract. It's a legal as well as physical arrangement set up to benefit & protect both parties. That was always the case, it's just that not a lot of people were willing to call a spade a spade. I think it would be good for you ALL to know what this "marriage" is really going to mean to the married couple.
You also mentioned that the prospective husband "has no issue" with her having relationships with other "women if he's not around" (whatever THAT means ?) . The implication there being (I may read this wrong ?) that he MAY have "issues" with other males in the picture. You really need to know that - ya think ! ? Everyone does ! From the heart - honestly.
So it seems you're in the position where two outcomes are most likely.
1> You all sit down & have a real heart-to-heart which may affect the plan for any marriage.
2> You just sit tight on the sideline and accept that the marriage MAY need to go ahead for those "other issues at play" and that if that's what's most important to her it will end up excluding you. At least in any honest and drama free way.
I doubt that anyone here would propose any solution to pursuing this relationship with less than 100% openness & honesty. Too much potential harm in that path.
And whether we like it or not "marriage" is a common tool for meeting people's physical (security, financial etc) needs, even if not their emotional ones. People are willing to make that trade-off. We all wish it could be otherwise - but it isn't. We need a roof over our head, we need to eat, etc etc. I think that realization is substantially at the heart of the growing "poly" acceptance. There are ways we can find the pieces we want for our happiness & security, but in order to do that we have change our thinking, fight our insecurities and RISK.
Go slow, be honest - both with yourself & the others.