I have never known anyone in this situation, well anyone that has told me about it, anyway
Just because it wouldn't suit other people OP it seems to work for you - It would not do for me, however I respect you for taking such a considered approach
You keep saying you are happy as a couple, well clearly he is not - or he would not be risking all, for this other woman. He thinks that much of her that he is risking all for her - and that would be the part that would sting for me
and the fact that he is dipping his dick in someone elses vag, and then whacking it up yours. ewwwwwwwwwww its dirty, mate
The whole thing sounds like a financial arrangement as opposed to a marriage too in many ways
Yknow there are an awful lot of people who just stay together for the kids
One thing that gets me is you talk of the upheaval of a split, and i totally understand this - BUT you are leaving the ball in his court - everything is up to him, if he leaves later on when the kids are grown up, you have no control over any of it ..................and if you become single in ten years, you are less likely to find someone else
I am not saying you are wrong, just blathering on like an idiot really