Polygamy.....come get some?

[nonpareil]

There are big differences between the old and new testaments.The old testament mostly deals with c.1900bc to c.500bc and has some basis in historical reality apart from the origin myth of Genesis.Our world has been framed far more by the new testament-1st century AD.Christainity developed from a sect of Judaism which has many rules and laws but became close to its modern version in the 300/400s AD as for various reasons the old Roman religion was abandoned.The Christians distinguished themselves from old Roman ways by having one God and likely more strictures on sexual habits.This was also a time of upheaval so possibly a somewhat puritanical religion was seen as needed in order to "put the world to rights"again.So monogamy and sexual conservatism have early roots in Christianity. These things have come down to the present.Monogamous marriage will also have suited males with property through the ages, to pass on their wealth to their genuine offspring and to keep such wealth in their family line.Marrige in the past would also have been a way to stop the female straying and so cut out the possibility of a male raising someone else's child.As a species we have a tendency to monogamy anyway (even if not complete) since in days of yore if both parents stuck around more children would be raised successfully to adulthood. Also if living conditions in the past were harsh monogamy was likely the default position, easier for one man to support one woman than several.So for various reasons one man, one woman has often been the model adopted by societies.

[nonpareil]

Straining against monogamy of course is the genetic advantage for a male to "spread his seed".However even if having children with other women if given the chance, he may likely concentrate on the children with one woman.There can also occasionally be a genetic advantage for a woman to stray.She could get pregnant by what she or her body tells her is a high genetic worth male but return to her first mate to raise the child- he may be a better parent and less likely to stray himself and he might never know the child wasn't his but raise it as his own.