How is it that people can support same- marriage but at the same time oppose polygamy?

I think it is very hypocritical for persons to trip over themselves in giving a ringing endorsement for same- marriage while at the same time denounce polygamy. Remember, behaviour was once illegal and criminal also. Also same- marriage did not exist anywhere in the world prior to the 20th century, while polygamy has been around since civilisation began. In my opinion, this is nothing more than the West imposing their thinking and mores on the rest of the world.