No, it's not "natural" to mans nature. Mans nature is to be tempted and stray. But it was instituted for the greater good of society as a whole. The basic sexual nature of man is the very same as the basic sexual nature of most species of animals. A fact that is not very surprising if one considers him only another member in the family of animals. At any rate the best way to understand his sexual nature is to observe sex in his relatives, the animals. A typical example of sex among the animals is seen by observing the dog. The dog wanders about courting and engaging in sexual intercourse every female that appeals to him and returns his interest. Likewise a female will be receptive to any of many different males. After the mating the male dog leaves the female to pursue other females and the female is left to bear the young and raise them. The male spends his entire life pursuing females. It is probably fair to say that that occupation is the main driving force in his life, it is his great pleasure in life. It is almost his main function in life, his purpose for living, his great preoccupation. The female dog, however, is not so preoccupied with sex. Especially after the young are born she has the long, practical, mind-occupying job of raising those young and doesn't have time or inclination for the foolishness of sex. Thus the male and female's sexual natures are different --- the male is always looking, always driven by his sexual desires, and the female is only interested in sex at certain times. What we have said about dogs could also be said about cats, cattle, horses and most animals. And it is also an accurate description of the basic sexual nature of man. However, in man these basic feelings and inclinations are countered, thwarted and held in check by other forces. The forces that thwart and hold in check his natural sexual inclinations are things like reason, conscience, religion, social pressure, culture, fear of God, good judgment, and sense of right and wrong. But the basic inclinations and nature are still there, always trying to exert themselves. Because of his mind and conscience he is doomed to be a creature of inner conflict, a creature with two natures at war with one another, a higher nature and a lower nature. http://pages.prodigy.net/jmiller.cb/a333.html Here's some information regarding the institution of marriage and it's purpose, if you're interested - http://en.wikipedia.org/wiki/Marriage

Its understood that the two should remain monogamous.

whats natural is further production of the species penguins and a couple species of parrots, and chinchillas mate for life (and im sure a few more species) but we arent one of the species that do that naturally but i think thats part of what makes it so special you love that person sooo much that you go against nature