Man and woman as created by God did not appear on the planet instantly, like magic, but emerged out of the animal kingdom which means that human nature embraces animal instincts and behavior which became humanized, that is, natural instincts came under the sway of their intellect and free will.
Do we find "monogamy" in the animal kingdom?
In birds we find a 90% rate of social monogamy meaning that birds mate for a season, several seasons or for life. On the other hand, we also find in birds a high rate of sexual promiscuity. Social monogamy is dictated by the requirement of procreation where both male and female may engage in nest building, incubation and feeding the young. No doubt, seasonal monogamy may be dictated by the average short life of small birds in the wild, who if they survive the first year, live between 2-10 years.
In mammals, there is only 3-5% that mate socially for life. The primates, that is the group of monkeys out of whom mankind emerged exhibit this trait. "Monogamous groups consist of an adult male and female with their children. When they are grown, the children leave to create their own nuclear families. ...It is found among the small Asian apes as well as some of the New World monkeys and prosimians. Specifically, monogamous family groups are the common pattern for gibbons, siamangs, titi monkeys, indris, tarsiers, and apparently some pottos." (Dennis O"Neil) The small number of young, the long period of infancy, e.g. gibborns reach maturity in five to eight years and the need for protection, were all factors which lead to the need the monogamous family. And so when the human species appeared, they already had the instinct toward the monogamous family and, as matter of fact, in all primitive people still living today we find the monogamous family unit. This does not mean that they are sexually faithful to only one male or one female.
But man, who, from the beginning was embedded in the natural order of things, was created in the image and likeness of God. They were created with the gift of integrity. In this original condition monogamy, sexual and social, would have been de facto realized. Today, however, in the U.S. there is a 50% divorce rate which indicates something went wrong. When our first parents sinned, they lost the integrity of their nature. Their nature which had been elevated above the animal kingdom now fell under the sway of animal instincts. Of course man still has free will but the exercise of it over his lower nature became difficult. Christ was once comforted by the Pharisees who wanted to know if divorce was lawful. He told them that Moses allowed divorce because of the hardness of their hearts but is was not so from the beginning. God intended the relationship of man and woman to be monogamous as we read in Genesis, "Man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh." (Gn 2:24)