A little pessimistic here..sorry.
I don't know when you are going to get married but might I suggest a very long engagement; at least until you see how you deal if this other relationship does happen.
I'm also a bit sceptical when people who all of a sudden feel polyamorous because they fall in love or want to have sex with another person. Sometimes people actually lose connection with their partner but don't have the strength to deal with it…that's what happened for me. Poly can be wonderful but it can also be used as trendy cover to mask some pretty basic human behaviour.
Be careful...think about all aspects of your future and definitely ask both of them very hard questions. You're talking about getting married; marriage sets couples apart in a very integrated way. Would this other woman be ok with that? Will you be ok telling your bridesmaids about your husband's girlfriend?
Protect your heart, be understanding that this could be very real for your fiancé and think further down the road. You obviously want a long future with this man so looking long term is highly recommended.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over