"In theory there is nothing to stop that definition being changed to allow group marriages or any other weird set up that some minority demands......
Before long the institution of marriage would become meaningless."

Why, no- not 'meaninless' at all...just a more complex and inclusive meaning! Why should marriage be allowed to mean only "what WE get to do in our narrow little concept of joining loving people in a legal union"?

If you want to invent terms to designate the differences between M/F marriage, M/M marriage, F/F marriage, just so you can write about it, be my guest...but if it's a legal, binding, state sanctioned ceremony with all rights enjoyed by M/Fs, a few quaint, whispered words will probably be tolerated...just as they are now about the NON-legal relationships.