Some free campaign advice from me.
If one of your major arguments against same sex marriage is that it will lead to polygamous marriage also being made legal, it’s not a great idea to state that polygamy is more justified than same sex marriage, as a natural institution.
Interesting of the three sorts of polygamy, polygyny is most common – a man with multiple wives. As Liz points out it is already legal in around 60 countries. Polyandry is not really legal anywhere and exists in a just a few far flung places.
Also of interest is that the Nazis looked at making polygyny legal because WWII killed so many men. Their reasoning was that with such a shortage of men, some men have to have multiple wives so that all German women could have children.