Polygamy & polyandry. Why are they illegal in the US?

Polygamy & polyandry. Why are they illegal in the US? Why is gay marriage such a hot topic and polygamy and polyandry not even thought of as part of a debate on marriage? What makes gay marriage and polygamy so different? Should polygamy and polyandry be legal? Is it legal in any other countries? Since this is a social science class there is no right answer. It also means that some personal reflection is appropriate but a significant amount of the paper should be based on reliable outside sources.