It was a method the romans merged different societies to itself to propagate its own growth (financially intellectually or geographically). In this merging they absorbed the good of the societies, and tossed out the bad.
Taken to further growth by christians in order to.. well do lots of things. Destroy the pagans, create a dissected caste system separating the pure bloods from the poor folk...or just in general, a method of pure unadulterated control over everything (including the monarchy) until a certain king challenged the concept of divorce.
In history you most often find matrimony being monogamous, but mating being more, liberal. Look to the romans, english, greek etc for history on this. Being married didn't preclude having some side action.
Monogamy grew with christianity (and western dominance) as did the laws of most western countries...
Thats the bare short of it of course. I am leaving out a lot of details I am sure others may be more interested in listing haha.