If you toss aside that foundation and say, "Let's add something more" then it's not a tweak to an old institution. It's a complete destruction of the foundation of the institution. It's not a slippery slope; it's a cliff. Marriage between two men? Fine. 3 people, four people, animals, siblings, little children -- upon what grounds could anyone say any of that was somehow "wrong"? We threw away the foundation and now all bets are off.
1 man, 1 woman. Or else absolutely anything goes.
Good point!