Consider:
Traditional marriage, is based on the union of dissimilar units, namely, gender. That is the qualitative property of marriage, and since there are only two genders, male and female, then the qualitative infers the quantitative, which in this case is two.
Homosexual marriage is not based on dissimilarity, but rather, similarity, therefore the qualitative property of this type of marriage necessarily infers a variable quantity.
You can't alter the qualitative property of marriage without also altering the quantitative.
Did you think that up yourself? It makes too much sense! /sarc