There isn’t anywhere in the Bill of Rights or Constitution that says the government may write laws regarding marriage. I’m against them marrying as well, but the federal government has no legal authority on the matter, and as such all laws passed by them should be null and void, as they are unconstitutional.
The federal government is in charge of writing the laws in regards of marriage, for it's own dealings with the matter, so they do have some areas where marriage comes up, for instance, the military, immigration, federal employment.