Marriage was a religious thing but not anymore. That part is optional. If one wants to become a muslim and indulge in polygamy, fine. I don't care. But don't have those people come around and demand that such behavior be rewarded with the same state benefits as the normal kind of marriage which is known to be a beneficial one for society,as a whole. The normal marriage is the foundation of our society and it will always be for obvious and logical reasons.
And yes, while it is true that there is no way of stopping people to be involved and live with multiple partners in the same house, and give into depravity, that is not the point. The point is not to reward this behavior and consider it on the same level with monogamy because it is clearly the inferior option, proven by society and history.