Because of the cultural changes in sexual roles and child protection and adoption--most especially since the 1960s and even earlier--all the reasons the state should involve itself in marriage have been eroded away.
At the time when each cultural change was foisted upon us, the harm in that change was too complex for the average person to perceive, and so the change was effected.
Frogs in water brought to boil:
Soon the average schmuck will not be able to logically object when the liberals propose there's no reason for state-supported marriage of any sort to exist any longer . . .
And the liberals would be right, because liberals have been systematically removing one good reason after another until there's soon to be no logical reason for state supported marriage to continue.
After all, if everyone is allowed to to their own thing for however long and wherever they want to do it with whomever(s) they feel like--how does the law make and enforce rules for that?