for myself (and most) the thought of my wife having sex with anyone but me is horrifying, repugnant and fury-inducing.
So if your idea of a happy long term marriage where we love each other incredibly deeply includes sex outside the marriage (i.e. swinging and similar) I'm going to struggle to see your point that the pair-bonding mechanism hasn't been broken. A totally non-negotiable element of the pair in my book is complete exclusivity.