kane,
I will explain using unrelated but more vivid example so that you could understand how it devalues marriage.
Lets take word "automobile". We all know what it is. Then suddenly somebody adds new definition to "automobile" and starts calling this thing an automobile as well:
So now when you say "automobile" it could mean actual automobile or that carriage. Word "automobile" is devalued.
My point is that there is no need to add WRONG definitions to the word. Why not simply invent new word, for example "garriage"?
So "marriage" would be union of man and woman and "garriage" would be union between same sex. Everybody would have equal rights, just name of the union would be different.