You guys know you can just TALK about this stuff for a while, right? Nobody has to jump in the sack with anyone else just yet or rush into threesomes?
Nobody ever died from taking it easy and going slow. Where's the fire? What's the rush?
Spend some time reading together and learning what you both envision and see that you speak the same language about what sorts of open model relationships
appeal and do not appeal. Do you even match UP in your polyamory style? Just being open to polyamory in general alone means nothing. What are the details? Expectations? Wants? Needs? Limits?
Maybe these could help you talk to each other?
If this is going to be part of your married life -- you might consider a long engagement too to help talk out "future building a life together" kinds of goals. How does raising poly children (if you want kids) play into it? What about sharing family holidays with your poly peeps? Time management? Financial resources? Other?
If it doesn't work out, it's easier to undo an engagement than a marriage. So don't rush into poly or marriage here. Both are major undertakings. Do you know the stress scale?
Some things in life just happen. We do not choose when. We CAN choose WHEN we open up and WHEN we get married though. Don't pile stuff on all at one time.
Go into this with full information and eyes wide open. It can be a lovely thing, or it can be a messed up thing. It's all on you guys and how you choose to navigate.