defining 'redefining'

I've heard from some equal marriage opponents that they are opposed to "redefining" marriage. What does that mean, exactly? Redefining the word? I'm sorry, that's not how language works. Parliament or whatever doesn't get to change the meaning of the English language word "marriage". That has already changed, by the mere fact of lots of us having talked about the concept of equal marriage, without a single jurisdiction needing to have changed its law to recognise such status.