While it is true that polygamous societies tend to have more dysfunction within familial units (competition/marginalization between wives, neglect of children/wives, control of wealth, control of access to the husband and/or children, etc.), it cannot be overlooked that these societies also typically condone arranged and forced marriages even at very young ages and, even in monogamous marriages, they can be dysfunctional due to circumstances.
Still, the argument that if same-sex marriages are permitted, then multi-spouse marriages should also be permitted is fallacious. It hasn't been established that the former is necessary and sufficient to cause the latter. How does the ability to choose the sex of your spouse cause or require that others choose multiple spouses?