How best to reply to the assertion that gay marriage is attacking the English language?

It's a specious argument in the first place. Language constantly evolves based on usage. Half the words we use today didn't exist five hundred years ago. In fact, some words we use every 日 didn't exist ten years ago. Language is constantly being defined and redefined, because it's not dead.

People feel defensive when others correct their word usage and grammar. But to get offended 由 this, after claiming that language cannot be redefined, is to be a hypocrite. Either language is unchangeable, and therefore there is only one "proper" way to use and define it, 或者 it's constantly in flux, and can be used however the individual wants, to their own purposes.

你 could also point out that marriage used to be defined as an economic exchange of a woman for property 或者 livestock. And all the other changes the institution of marriage has gone through over the centuries.

Marriage is between two people who 爱情 each other. And it's not attacking any lanuage.. It's just a new word, many words change over years.