Imagine, there were all same sex marriages on earth, and visualize its devastating results, with no babies, no new generation, no new life in continuity on earth. Therefore the nature has put up limits on certain things.

So, the same sex marriages are unnatural, because the nature has created the practices of life on a continuity basis.

On the other hand, Islam, which is a religion of "Nature" has allow men to marry with more than one wife, because the men's are prone to earlier deaths, as compared to widowed women and higher quantity of women, created by the God.

So, polygamy is, in fact, a product of "Nature" with natural benefits to the mankind.