There is a vast difference between Holy Matrimony, and marriage.
If you need, I can explain it to you.
If we proceed under the assumption that government has a role (which I'm not sure I agree with), then I posit that you don't have to support the inclusion of polygamy. It would be the responsibility of those who would exclude it to say why government would have such power.
I await those arguments, but the people who address it just keep asking questions.