Basically, it requires both partners to be able to identify that they only control themselves, not the other partner. If one is 'highly' monogamous ( spouse shouldnt even think about another partner. Add in highly religious, fidelity of the mind, body, and spirit.) and the other is 'highly' polyamorous ( Wants to love everyone, and doesn't believe in any type of rating, or withholding.) It will be tough to find middle ground.
If either of these types needs the other partner to be absolutely alike, then there will be trouble ahead.
My husband has a 'take it or leave it' attitude about any non-monogamy. He has fun, but he wouldn`t ever go out of his way, to attract others. He would be happily monogamous.
I am just on the other side of that. I like being non-monogamous, prefer it, and seek it. However if I needed to be ( I have in the past.) monogamous, I can do it. I don`t 'need' more then him, I just 'like' more then one.
So,even though we are on slightly different sides of the same scale, we can give and take with each other pretty easily.
The further away each of you are from the mid-point, the harder it will be.