Well, as I said a bit earlier up - all three concepts came into the Law together. The Federal government has the power to regulate marriage - but it doesn’t have the right to define it. Same as with Habeaus Corpus and Trial by Jury, fwiw.
But if they remove marriage from the equation, how is that trying to define it?
Right now when couples separate there's a legal mess (particularly if children are involved). That legal mess doesn't change whether or not there's been a marriage. There are still questions of support, property separation, and child custody.