On the one hand, citizens continue to be free to move to whichever state best suits their chosen lifestyle.
Unfortunately, there's Article IV, Section I, where each state has to recognize the public acts, records, etc. of the others. I think that's what Congress was trying to solve with the Federal Marriage Act (or whatever the current law is called).
Regardless, any such "Federal benefits" you have in mind, e.g. Social Security benefits, are likely unconstitutional. That's the real problem coming back to bite us on the backside. If this nation hadn't strayed so freakin' far from what was intended, "Federal benefits" wouldn't be an issue.