The U.S. Supreme Court must rule the federal Defense of Marriage Act unconstitutional as a matter of states rights.
The federal government has always allowed states to set their own definitions of marriage as long as they are not deemed to be discriminatory. Those states that have granted same-sex marriage have by historical precedence the right to set their own rules regarding marriage. States that want to ban same-sex marriage have the right to do so. But by federal law, all states must recognize marriages from other states.
This must be upheld at the federal level. All federal law must be applied equally, and no group of citizens should ever be excluded from equal application of the law.