When anyone can marry anyone, and in any number or combination - then marriage effectively becomes meaningless from the standpoint of the state. The next step is to ban marriage as a legal institution.
That would be a huge step in the right direction. Not a ban on marriage per se, but a complete eradication of any government involvement in the institution of marriage.
There are very few things more indicative of a totalitarian state than the simple concept of a marriage license. Who the hell is a government to order people to get a "license" to be married? That's the equivalent of a government demanding a license to be out in public while you are black.