Nope. Dawn of civilization marriage thru the 20th century, limited to men and women because only men and women procreate, the exclusion of closely related people is perfectly constituional. From BC Roman law
Aside from the fact that this has been proven wrong repeatedly, ancient law does not form the basis of modern US law. While that is not to say that ancient laws, religious laws, and foreign laws do not influence what we pass, those things are not our basis of law, nor do they form the foundation of whether or not something is constitutional or not. The Constitution does that. And even that does not in and of itself guarantee that rights will not be violated, as examples by Prohibition. It has also been further pointed out that, based upon how legal marriage is structured in the US, (not social, not religious), that it is more constitutional to ban procreation between closely related people, than it is to deny them the legal institution of marriage.
"matrimonium was then an institution involving a mother, mater. The idea implicit in the word is that a man took a woman in marriage, in matrimonium ducere, so that he might have children by her."
Another point on which you have been balked. For that basis to be legitimate on the institution, the basis for the word in all languages would be based on "mother", which it clearly isn't. I provided the link that shows both matrimony and marriage in almost all languages and it shows clearly that "mater" is not the basis of all those words. Unless you can show where those words are derived from the use of the other languages' basis of "mother", your argument fails. Additionally, while it was assumed that children would occur by most male/female marriages, those offspring were not initially the basis of marriage. Marriage was about power, wealth and/or alliance. Only recently, in the historical context) has marriage been about love and not these things.
Mater semper certa est ("The mother is always certain")
"pater semper incertus est" ("The father is always uncertain")
"pater est, quem nuptiae demonstrant" ("father is to whom marriage points")....
This only point out probability, alongside a willing blindness to affairs.
to 20th century marriage
Sec. 160.204. PRESUMPTION OF PATERNITY. (a) A man is presumed to be the father of a child if:
(1) he is married to the mother of the child and the child is born during the marriage;
Again, assumption of a father is not automatically a correct assumption. By this a woman can get pregnant by her first husband, divorced or widowed before birth, married shortly after that (again before) birth, and then a man obviously not the father, legally becomes the father. Furthermore, especially since marriage is not required of procreation, presumption of paternity has no bearing on the legal institution of marriage. If anything, it is the others way around. Indeed the presumption is based upon the marriage, not marriage based upon possible or potential paternity.
Perfectly rational to exclude the closely related. But youve adopted the charade as your reality. That marriage never had anything to do with procreation and this 1000s of years old limitation to men and women was all nothing but a nefarious plot to exclude the homosexuals, motivated by animus towards homosexuals.
About as much a "nefarious plot" against homosexuals as there were nefarious plots on excluding left handed people. I really doubt that anyone on either side has assumed any plot in the natural human tendency to exclude the different. It's a commonly repeated theme. Any limitations have been about control the fears of those in control, as demonstrated by the marriages that were other than the common. Historically speaking, more marriages were simply a declaration of a couple that they were now married, either by their choice, or by arrangement by their parents. Marriage has had a broad variety of definition of participants and the results of it. There is no one historical absolute. Which brings us back to simply what is law, with regards to the legal recognition of a marriage? And in the US under the Constitution, and by the simple fact that no legal marriage requires love, sex or children, limiting marriage such that any given adult pair cannot obtain that legal standing, is unconstitutional.
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