If still at least a century ago in society the conjugal ties were considered as something sacred, then in the modern world moral foundations it is much simpler. Sexual revolution made the business. For this reason such phrase as "open marriage" surprises nobody any more.
Legal side of a question
Marriage is regulated by normative legal acts. First of all, it concerns property and legal relations. The sexual relations do not enter here, and depending on any given public and religious foundations, approach to them can differ in various cultures.
Quite so the spouses living in open marriage consider: their union is officially registered, they in common make the decision on everyday issues, education of children, enhancement of a private property, however there is a huge amount of nuances which are a private matter of everyone. Including the sexual relations on the party.
It is impossible to call actually this type of the relations new. It was rather widespread many centuries ago, especially among rich layers naseleniyeya as public foundations obliged to choose to themselves the partner in life among people of the circle. Such spouses lived together, accompanied each other on public actions, brought children, but, in addition, each of them had the private life. The main thing – that each of them felt most comfortably.
Psychological aspect
Some initially aspire to the open relations, others come to it in the course of family life. The similar behavior is explained by desire to keep privacy, personal space, but at the same time to have family and children. Besides, according to supporters of open marriage, it is much simpler to build life with the same person, affording some liberties on the party, than every time to get divorced and enter the new relations based on fidelity canons. It is only a way of existence which everyone chooses for himself. In modern society often open marriages are entered by wealthy people, businessmen, the bourgeois. There are such couples and among show business stars. The most striking example it is possible to call marriage of the actress Monica Bellucci and Vincent Cassel, Gwyneth Paltrow with Chris Martin. There are families which during a certain period pass to the relations as open marriage. Fatigue from each other affects, more often there are conflicts and scandals therefore on family council the decision some time is made to live separately. After all it is better, than to divorce, without having managed to consider everything. According to a number of psychologists, the similar respite will help to understand itself and to understand whether really you want to continue to live together, or it is better to leave forever.