Define legal domestic partnerships as between "consenting adults" and give them all the same rights as what "marriage" has today.
With this, "marriage" is no longer recognized as a legal partnership as will not be given any legal benefits. Religious ceremonies can still be had if the individuals chose to, but it means nothing until a legal domestic partnership license is granted.
By defining it as between "consenting adults" it gives way to heterosexual, homosexual, polygamous, and platonic domestic partnerships who simply want the legal benefits of being legal partners. This also eliminates the common polygamous practice of marrying children, as they are not adults and not allowed to be in a legal partnership.