It's inevitable we can't write out our whole stories, esp when things are moving quickly
I was just curious because I had a chain of events somewhat similar to yours. My partner & I were open to having other relationships, but hadn't met anyone else we wanted to be with. Eventually we realised we wanted to live together, and decided to get married (well, civil union-ed). We were neutral on the official marriage part, but knew both our families would prefer it.
After we'd decided this, is when I met someone else, and it quickly got serious... this was our first proper poly experience. It was interesting dealing with the newness around that, and everything it stirred up, in combination with wedding planning. I'm pleased to hear you two made it through as well! Many happy returns