it seems as though you are seeing the concept of marriage as a sort of absolute truth

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.