It won't end civilization but it will end the institution of marriage as we know it. Whether or not that will have an effect on civilization is for each of us to decide. I, of course, have decided it will have a profoundly negative effect and thus I will oppose any redefining of the word.
The upside I guess is that I will get to see the dire predictions of the downside of it all, refuted in my lifetime. And when we are both collecting social security, I can say, see I told you so John.
And we can chat about whether the two remaining justices on the otherwise vacant SCOTUS bench (Thomas and Breyer), will deadlock yet again.
And there you have it. :)