Marriage is the word we have used to describe a very particular form of relationship. If you try assign the word marriage to something else it can’t keep the original meaning.
Who recognises marriage does not change fundamentally what marriage is.
The word “retard” did not change meaning but the reaction to using it did. This is a symptom of political correctness, since “retard” was the politically correct umbrella term. There are some other words that have had the same treatment.
Though there are backwards cases where children are better off with other people than biological parents, there are specific factors leading to this and the overwhelming reality is that biological parents are far better.
Regarding marriage keeping its original meaning - it seems as though you are seeing the concept of marriage as a sort of absolute truth rather than what it is - a social construct. Any concept created by humans can also be changed by humans. At this point in time, it doesn't make sense to hold on to an outdated meaning if that meaning no longer applies to the current reality. It makes more sense for the meaning of words to evolve along with the reality of the world.
Who recognizes marriage actually does fundamentally change it. Words hold meaning only because humans give them meaning. Words in themselves are nothing more than a symbol which overtime, comes to represent a concept. Concepts are held in place by an agreeance of individuals. However, as the individuals who maintain the concept change their understanding of the concept, we see a general paradigm shift towards a new meaning. That is how science and philosophy and ultimately human society, operate.
As already stated words change meaning all the time:
"Gay" used to mean "happy," now it means "man who has sex with men."
"Sick" used to mean "ill," now it means something is "awesome."
"Cell" used to mean "jail" or "body component," now it means "phone."
"hook up" used to mean obtain a service, now is means to "have a sexual encounter with someone."
and so on and so on...
To say that words cannot change their meaning is nonsense really.
Definitional truths are absolute.
Your examples are not very good because the meaning is still there in most of them.
Gay still means happy, but has a homosexual connotation due to the fact that gay men are more openly whimsical and cheery.
Sick still means ill. There was a period where people used it to mean something completely different, and this is one of the few valid examples, which I will address later.
Cell means the same thing. A jail cell, a body cell and a cellular phone all use the word in the same fundamental way.
Hook up is in regards to communication, and so the sexual connotations are an implication of the communication.
The word sick was used differently in a particular type of culture. This did not alter the meaning of the word, but created a new meaning relevant to the specific culture. Altering the meaning of a word is not the same as applying the word to a different situation. You could create an additional definition for the word marriage but the fundamental relationship will not change.
The reason marriage cannot be redefined or undefined is because the word marriage, which doesn't need to be the word marriage and could be the word snurbleburp, refers to a very specific type of relationship. This relationship could be called anything but it remains the same thing. Whoever wants to recognise the relationship does not alter the relationship. You could use the word marriage to mean whatever, but you can't redefine the relationship.
The misunderstanding between us is that you were under the impression that I was referring to the word, not the meaning. I am referring to the meaning, not the word.