Can sisters marry?
Given the left’s arguments, there’s no reason they couldn’t.
This is the real problem here. One of the Justices' today brushed against it.
If we redefine marriage to include same-sex couples; then, what moral or legal argument is there to not re-define it again for group marriages; or, two girls and one guy "marriage". There isn't any, because once we redefine marriage, it is open to be redefined over and over again. It will be redefined to whatever combination of people (or animals?) that can be thought of. It will dilute the meaning of marriage to a point where it is meaningless. And, that is the point of this whole exercise.