In all the cases you list, the question rises, why is marriage the only option?<quoted text>
The gist of what you are saying is that marriage is a worldwide practice that was invented by mankind to hold one man to one woman for the sole purpose mating and the continuation of the species.
And you and I both know this isn't the case.
I've listed the multiple reasons that people get married. Many of these reasons have nothing to do with mating or continuing our species.
--An 80 year old rich man who marries a 20 year old bombshell (trophy wife) may have no interest in reproducing.
--Two elderly people who get married--either widowed or divorced--may not want to mate or be unable to mate or reproduce.
--Two divorced couples with children may marry, but decide that they have enough children and are not interested in creating more.
--A couple in which one is infertile cannot mate, reproduce and produce an offspring.
--A couple who marry and simply does not want to produce offspring (one person may get a vasectomy, the other may get tubal ligation.
These are largely Western examples of not getting married where the sole intention is to reproduce and continue the species.
In China and other parts of the world, "ghost marriages" are performed, in which a deceased person is married to a living person or two deceased people are married to one another at some point after the death(s) have occurred. Obviously, this kind of marriage doesn't involve reproduction--doesn't involve the continuation of the species.
Marriages in many parts of the world are simply property based. In other parts of the world they are performed to tie two tribes together or to settle disputes between two tribes.
And as we all know, the "constraint argument"--the one in which you claim marriage "constrains" the man from marrying others, is not practiced. Many cultures practice polygyny or polygamy. Many cultures may practice a "monogamous marriage", but encourage or allow for mistresses.
So, you see, your "quote"--the one you won't reference--is clearly flawed. It simply is not true.
In your favor, many people do marry in order to create a family. But not all do this.
Your quote should read, ""Marriage is O-F-T-E-N a cross cultural constraint on evolutionary mating behavior. However, there are other reasons for marriage; many that do not place the same constraints on couples and some that are not for the purposes of evolutionary mating behavior."
I still am very interested in where you came up with your quote. At this point, I'm pretty sure it's something you created yourself and not a "social science fact" as you previously claimed.
Answer; Because of the cross cultural constraint. That constraint is rooted in procreation. Yet, it's influence establishes a cultural expectation of marriage.
Or put differently, if there was not the ponderous propensity for children, marriage would not have the cultural hold.
As to the basic essence of marriage; a cross cultural constraint on evolutionary mating behavior, all you have to do is disprove it.
Maybe if you study mating behavior, you might find a better definition?