One of the things I am struggling with myself is that even if we did nothing our relationship (me and wife, fidelitous and mono) could end; we can change and grow and need without the addition of others.
So take that as the basis, that both of you will have to grow and change as you age, and ask a different question:
1) Why do you think you will have a lifetime together without being poly?
My wife and I have both expressed a desire to spend a lifetime together. Unfortunately the way genetics work, that means only 40 years. She may be luckier and get 50 years, meaning the last 10 years will be without me.
I don't think that's driving us, but it does influence my thinking, as I have already lost both of my parents and am constantly reminded of how short life is when I see my children grow up in front of me.