This is going to be deleted anyway because the thread was cleaned, buuuut:
The US was built on Christian principles. With that in mind, this makes a fair amount of sense. Other countries are built on other principles, and that's perfectly fine. Also, the man/woman definition has nothing to do with why polygamy is illegal. After all, polygamy could be man/woman/woman/woman, eh? Gays can get all the marriage rights they want, polygamy is still not going to be legal. Why is that specific instance even being brought up by you so often? Can you not decide between two potential spouses and this is how it manifests? =P
The U.S. also supposedly allows for freedom of religion and supposedly believes in the separation between religion and state. At least, that was the idea.