Why monogamy became the social
Family, and she is considered to be the cell of society rests not just on a monogamous relationship. Moreover, it is monogamy, how do you think evolutionary scientists, was one of the reasons that man became bipedal.
The story developed as follows. Primates, the ancestors of man were together for a short relationship, after which the female remained with the offspring, and the male went in search of a new relationship. But some couples tied to each other is stronger then the male remained with the female and helped her to take care of the offspring. He went in search of food, and when he found her, then brought his family. But it turned out that in the mouth can bring quite a bit, and the monkey then ran on all four paws. Then some males had thought to bring food in then still not issued the "hands" moving in the lower extremities.
In traditional Western society monogamy was fixed very firmly. Even today, when women are able to get food, even when children are small, most of these concerns fall on the shoulders of the father. This necessarily implies that the monogamous family. Otherwise, why would father care about someone else's offspring? This approach strengthened the position of monogamy in ancient times, although now it often happens that a man takes other people's children as their own. But today, a monogamous relationship is considered to be ideal for raising children.
However, in Eastern countries men are allowed to have several wives, as Islam permits such things. In countries with a Western mentality (Europe, America and Australia) are allowed only one partner in the marriage, and it is sometimes even enshrined in law.
The opinion of psychologists
Despite the fact that monogamy is the accepted way of marriage, infidelity among spouses is very common. Psychologists believe that people have to "Mature" for monogamous relationships, and it is not easy. The fact that people may have deep feelings for one person at the same time, sometimes "carried away" by someone else. Passion passes quickly, but in this period the probability to commit treason sharply increases. It also happens that people change in the heat of passion, for example, during intoxication. Much less likely to capture cases of long-term infidelity when a person has a marriage partner, and another, illegitimate, for a long time.
The fact that people's feelings is a complex and intricate system, sometimes quite contradictory. Not everyone has the strength to resist charming the opposite sex, even if you have a permanent partner. Some people have low morals, they don't even try to resist, sometimes even specifically hunting for extramarital "adventures".
There are those who believe that monogamy is contrary to human nature. Such people have relationships in advance of agreeing with a partner on this situation. Such marriages cannot be called traditional, but sometimes they exist quite successfully. Such an approach to the relationship is called polygamy.