God Almighty has created man as polygamous minded ,for that matter 99 % of the male species in nature are polygamous. In humans, female babies being stronger have a better survival rate than males... also historically, naturally and traditionally men go out to fight in the battle fields so have a higher death rate than females... Due to these and may be some more such reasons that today the female population of the world is 51% and men are 49%.... in some countries this ratio is much higher in favor of females.. probably this may be the reason that Almighty God created man as polygamous minded to marry upto 4 females to compensate for the numerical shortage of men. Now if the culture and traditions restrict a man to only one woman , then it's quite possible that he under the compulsions of his polygamous nature , tend to establish secret relations with other women.... and thereby cheat on his partner.