It is possible for non-citizens to immigrate to the US either on a marriage visa or a fiance visa. Regulation of immigration is a federal power and responsibility. I do not favor extending those visas to homosexual couples. I also do not favor allowing polygamists to use those visas either. I really don't think it is possible for the federal government to not be involved in the issue of what is the definition of marriage and who can be married to each other.
You should read about the debate over the admission of Utah as a state. It was precisely the issue of polygamy that caused Congress to delay its admission. This was in part due to the full faith and credit clause of the constitution. The existing states were concerned that they would be required to recognize polygamous marriages recorded in Utah. It was not until Utah territory banned new polygamous marriages that it was allowed to become a state.
You can bury your head in the sand and claim that each state should be able to decide this issue, but the advocates of "gay marriage" are bent on using the courts to force all states recognize them. If the Supreme Court votes to force recognition of "gay marriage", it will be the Dred Scott decision of the 21st century.
No, it won't
There will be no nationwide military confrontation over gay marriage.
Some screaming matches, perhaps