Many invalid, inflammatory arguments are made to try to justify continuing to deny gay people the right to marry. The argument that if same-gender marriage is permitted then polygamy will also have to be permitted is one of these. Polygamy is multiple spouses. It bears no resemblance to monogamy, regardless of the genders of the spouses. The purpose of claiming that the two are congruent is to confuse and demonize gay people. It’s a common device used by bigots of any variety. As a gay man, I could list dozens of these things. In my opinion, it isn’t worthy of too much energy because it is irrational and deliberately derisive.