If even a century ago in society, marriage was considered something sacred, then in the modern world the moral foundations are much simpler. The sexual revolution has done its job. That is why such a phrase as "open marriage" no longer surprises anyone.
Legal side of the issue
Marriage is governed by regulations. First of all, this concerns property and legal relations. Sexual relations are not included here, and depending on certain social and religious foundations, the approach to them may differ in different cultures.
This is exactly what spouses living in an open marriage think: their union is officially registered, they jointly decide on household issues, raising children, increasing private property, but there are a huge number of nuances that are a personal matter for everyone. Including sexual relations on the side.
In fact, this type of relationship cannot be called new. It was quite widespread many centuries ago, especially among the wealthy strata of the population, since social foundations obliged them to choose a life partner for themselves among the people of their circle. Such spouses lived together, accompanied each other at public events, had children, but, in addition, each of them had their own personal lives. The main thing is for each of them to feel as comfortable as possible.
Psychological aspect
Some initially strive for open relationships, others come to this in the process of family life. This behavior is explained by the desire to maintain privacy, personal space, but at the same time have a family and children. In addition, according to supporters of open marriage, it is much easier to build a life with the same person, allowing yourself some liberties on the side, than to divorce each time and enter into new relationships based on the canons of fidelity. This is just a way of being, which everyone chooses for himself.
In modern society, wealthy people, businessmen, and the bourgeois often enter into open marriages. There are such couples among the stars of show business. The most striking example is the marriage of actress Monica Bellucci and Vincent Cassel, Gwyneth Paltrow and Chris Martin.
There are families that, at a certain period, switch to open marriage relationships. Fatigue from each other affects, conflicts and scandals arise more and more often, therefore, at the family council, a decision is made to live separately for some time. Still, it's better than getting a divorce without having time to think it over. According to a number of psychologists, such a respite will help you understand yourself and understand whether you really want to continue living together, or whether it is better to leave forever.