Redefining marriage is revolution, not evolution

Should we, as a society, retain marriage as it has been understood until now, that specifically recognises the unique significance of sexual complementarity, which is not a doctrinal invention of the Catholic Church or any other religion but is the basic biological fact of nature, without which society wouldn’t exist because we ourselves wouldn’t exist?
Or should we, in effect, replace it with what amounts to a new institution, also called ‘marriage’ but inherently different in its nature from the present institution?