OneWithDunamis
Words change meaning overtime, but if we don't stick with the meaning that is currently found in dictionaries, what's the standard? Are we then free to say whatever we want and inject our own meanings into them? Are we also free to interpret what's presented to us as anything we want? The fact that we're responding to each other shows how important a commonly understood language is.
If you do not realize the contradiction there, I am sure you need to reevaluate your own argument. The standard is decided by what meaning is most prevalent. If a word has changed meaning and other people use said meaning enough, it becomes the new definition. The standard is how people use it, not the dictionary. You're arguing for keeping the word as is but fail to see that marriage itself has changed meaning from its conception:
Etymology Online
marriage
c.1300, from O.Fr. mariage (12c.), from V.L. *maritaticum, from L. maritatus, pp. of maritatre "to wed, marry, give in marriage" (see marry).
"When two people are under the influence of the most violent, most insane, most delusive, and most transient of passions, they are required to swear that they will remain in that excited, abnormal, and exhausting condition until death do them part." [G.B. Shaw]
Marriage counselling first recorded 1945. Marriage bed, fig. of marital intercourse generally, is attested from 1590.
Thus, marriage has changed meaning. Yet you're saying it shouldn't change meaning so we can keep it the same despite being able to change in the first place. If we go with what you said, we should revert the word back to this original definition and go with that. (This definition is rather sexually neutral so it would allow homosexual marriage). I have to go now and have fun proving your point.