Magic Man
Reaper of Conversation
Yes, people can do whatever they want to do, and meaninglessly and thoughtlessly use whatever word they want without making sense of anything. I grant that.
People can look at a bird and call it a tree.
Yes, another great example. That's exactly the same thing.
The point is that language changes. "Bird" will never come to mean "tree" for obvious reasons. "Marriage" coming to mean the union between two people is not much of a stretch. It's the same basic concept, just with a slight variation. A bird and a tree are two completely different things.
Why do you even use English? There are few words that mean exactly what their roots would imply? It might be better for you to just make up your own language and only use it yourself. That way no one will "ruin" it by letting it evolve.