Monogamy is actively encouraged by modern laws and way of life. Despite the fact that ideal couples exist only on magazine covers or in commercials, this is a stable stereotype. A happy marriage is a husband and wife who met each other on a school (student) bench, and their offspring. The third adult is undoubtedly superfluous. So the family is perceived in the mass consciousness. However, in life everything is not so unambiguous, and most of the marriages fall apart, and in other cases either "left" trips or serious novels happen. Do not go far: remember the love and family life of our great. Alexander the Second, Nekrasov, Pushkin, Tyutchev, Blok, White, Mayakovsky, Vysotsky ... Idyll was not at anyone. Monogamous faithful married couples are more an exception than a rule. For such a relationship model in which either two men share one woman (Block, White and Mendeleyev's Love), or the husband has, besides his wife, also a mistress (or a second spouse - for example, Ivan Bunin), there is even a term "Swedish family". By the way, absolutely it does not necessarily mean sexual adultery. The triangle can relate various relationships - both platonic and erotic, including bi-and homosexual (Henry Miller, his wife Jun and her lover). An important difference from treason is openness. Such married couples can experience a variety of feelings, for example, someone may seek to maintain a marriage and for this gives the spouse freedom (example: Mayakovsky-Lily triangle and Osip Briks). But all participants in such a union accept it at a certain stage of the relationship.
So, there are many examples. It is worth it to follow - everyone decides for himself. In such relations, people renounce the "ownership" of the partner, so in a sense, these are spirited unions. Discarding selfish jealousy, they acquire not only a new experience. Such triangles transform the psyche of each of the participants. After all, if there is competition, then it must be controlled. There may be an exchange of partners (the so-called "swing"). But more often it is possible to meet offers such as "a girl for a couple". And not always the initiator of such "bigamy" is a man. Often such a relationship is ideal for bisexual girls. Couples often look for not just diversity, but a full member of their union, with which you can share both everyday life and holidays. On dating sites often there are ads like "looking for a married couple" - and note, it's not just about sexual relations. You can evaluate this phenomenon differently, you can resent promiscuity or admire emancipation. But couples who decide on such an experiment are worthy of respect. By the way, in the eastern communities, polygamy is generally legalized - hence, has the right to life. From the biological point of view, such relationships also have their justification. Fertilizing several females, the male contributes to the genetic diversity of the genus. The most serious problems can arise in "wedding" couples, whom Orthodoxy or the Catholic faith prohibit unambiguously similar relations. And from the point of view of officials and legislators this is not an easy situation. Therefore, such married couples can not formally formalize relations with a third party.