Regardless, the subject of marriage is outside the scope of the constitutional powers of the federal government (and rightly so - I don't see why marriage should have been or should be added as an amendment to the Constitution. The rogue federal government doesn't need more power - it already has assumed way too much power illegitimately already.)
The feds didn’t have to perform marriages, just deal with defining marriage for the issues that it must deal with.
Now you are promoting the position that the feds will have to recognize gay marriage and polygamy, because the federal government has plenty of issues dealing with marriage, and as we see, always has, in fact legislating on it since at least 1780.