Is it possible that marriage, much like optimal diet, is different for everyone?
Sure, it must be so. Everyone is different, every marriage is different. I was talking in a general way, as marriage is usually seen as a contract of exclusivity, which ends up of being a kind of jealous possession of the spouse. This is not love. If you really love someone, you want that person to be free and happy, and it means that she or he can also be loved by and love someone else, in case it happens…
...and that happens always after some years of living in a binary couple. The common, “civilized” way of thinking is that the third party intrusion is going to break the initial couple and hence should be repealed. This proves to be a terrible mistake. The third party arrival, on the contrary may well be a good luck, bringing new energy and information into the initial couple – of course under the condition that all three accept each other.
Hunters-gatherers seem to know that since they gladly accept and welcome foreigners, even often to the point of intimately including them in their couples. That was still the way of life for peoples who had not yet fallen into grain and dairy consumption, such as Pacific Islanders and Inuits.
I believe you've mentioned before that you are divorced Francois. It's quite possible that this experience has defined your ideas of what is good and what is bad in a relationship. This is not to say that your particular experience must therefore preclude marriage from other peoples viable ways to express there love for each other.
Yes, I’m divorced. It may have influenced my ideas to some extend, but my case is not at all an exception. It’s rather the closed couples working fine that are an exception. After some years of living together in a closed couple, the relationship wears out, the passionate love of the beginning inexorably goes away and the couple usually and at best settles in a kind of affectionate routine. Some people may be satisfied with such a situation, which thus may last for all the rest of the live of both partners.
But some other people feel that something is missing in their live and they start to long for something else, something fulfilling their deepest aspirations. One day, they fall in love with a third party. That seem to be the normal suit of events. But unfortunately it looks like that since the advent of agriculture, adoption of a sedentary lifestyle with grain and dairy as staple food, at the beginning of the Neolithic era, something went wrong with the human sexual instinct and way of thinking. Private property appeared and it extended to the possession of the partner, the third party being ever since considered as an enemy coming to plunder the crop and steal whatever it could… including the partner.
The grain had to be protect behind ramparts, fortified cities appeared, everyone enclosed his property behind fences and walls. The tribal structure disappeared and was replaced by the family structure. Finally, chastity belts and other horrors such as stoning to dead for adultery were implemented.
Marriage is a neolithic convention. I do not believe monogamy is solely a neolithic invention. I believe the paleo era had monogamous and polygamous relationships and both can work.
Yes, to a certain extend. Monogamy can work fine for a while, and perhaps for the whole live in a few cases…
For the record I have been married for two years myself. There are times where my wife is a difficult woman to live with, as I can be difficult for her, but I feel our union is beneficial to the both of us. We both have a best friend, confidant, support base, and sexual partner who we know will be there for us when we need them. I am not so naive to believe that this would continue growing stronger, as it has since we first met, without lots of work and compromise. Nor am I so naive to believe that marriage will always work out between two people. I do believe it's an incredibly strong bond when nurtured by two people who belong together and I fully expect us to spend our lives together much the same as my parents have through all of their internal & external hardships (30 years in the spring). 
My parents lived together more than 60 years – till my father’s death. Marriage can work under some conditions, but it would work much more often and much better if it was considered as an open relationship, open to others in case of love with a third party.
Francois
