If you are asking for a suggestion, I would propose changing the definition from:
Marriage means the union of a man and a woman to the exclusion of all others, voluntarily entered into for life
To something like:
Marriage means the union two people to the exclusion of all others, voluntarily entered into for life.
This illistrates the revisionist dilemma. For every definition of marriage there is exclusion. By the arguments used here exclusion equates to "inequality" (which equates to slavery.)
It stands to reason that the only definition that would be acceptable is one that has no boundaries, whereby the definition of marriage is anything to anyone at any time to the exclusion of none.
Or no marriage at all.