That sounds fine if it works for both partners. I disagree with whether monogamy is natural or unnatural or whether that is even an issue.
Most of us choose to be faithful when we take a life partner and the expectation is that our partner chooses it as well.
Most of us don't feel the need for anyone to choose for us or tell us that our choice is unnatural
No offense but what you're saying sounds like a rationalization of behavior. If you have an open marriage and your partner agrees to it, that's great but I find it odd that you feel the need to bring the rest of us into it.