... a civil contract, determined by cultural attitudes. Agreements about the parameters differ throughout the world, generally dependent upon common moralities cemented by tribal rules of conduct for sexual intercourse. Where one lives (and learns) sets the standards. Enforcing the standards has become a function of government (neighbors), one of the areas that has unfortunately resisted separation of church and state. But, that is to be expected-religion feeds on the economic stability of government, that in turn collects taxes (and maintains civil stability) based on the religious standards of any particular society. If you don't believe in my god/s, thus not subscribing to my standards, too bad. You live in my house, you'll follow my rules.
In this era, transeuropean standards have generally been set by Christianity. Greeks and Romans were not so prudish as to forbid carnal interactions prevalent world-wide at the time of Yeshua of Nazereth-who must have reasonably been engaging in sex like all humans. It took 500 years for the Christian churches to begin enforcing who got laid by whom. It doesn't stretch my imagination to think Mohammet codified marriage based upon how things were in the seventh century, CE. There is an ancient understanding that we can "love" more than one person, interact (multi-tasking?) reasonably in a variety of methods with others.
Actually, carnal communion of sexes is dependent upon economics, always. It is the animal way ... get all you can within the means you possess. Survival of the fittest demands it! Money doesn't buy happiness, but it certainly buys orgasms.
Nature decrees mating timeliness, particularly for females. After menarche, all are liable to reproduce. And after menopause, most societies don't support the idea that females still like sex (if they ever did). The driving force of all societies is sexual. My paternal grandmother "took up" with her first husband at 13 (common in Appalachia), when first stirred to her destiny. My mother ran away from home with a sweet talking sailor. So do we become, for abstinence defies natural impulses..

"Marriage" is a device governing hierarchies use to attempt a balance of conflict in their respective cultures. Most individuals ignore rules when opportunities of convenience arise, for the prime directive of Nature for all species is: reproduce!

I think "marriage" is a poor system for control, and neither church nor government should determine what is authorized.Mating should be a method of effective partnership, a mutually pleasant interaction of individuals, however oriented. Social stability is developed from satisfied participants, not by edict of neighbors or family. I don't believe in prohibitions as a method of enforcement of standards. Education to necessity is a better way, leading to individual recognitions of discipline for harmony. If kids were taught that pleasing an Other is the best way to please One's Self, we might all be happier.
Hooray for "free love!" Live, and let live.