I don’t understand what any gay marriage ban has to do with the 5th Amendment.
Anyway, this is yet another attempt to equalize things by their nature unequal. The standard result is that the better thing (normal marriage) gets debased, not that the worse thing (homo-marriage) gets uplifted.
“The Government Accountability Office has identified more than 1,100 areas of federal law in which marriage matters, ranging from tax and welfare benefits to employment and immigration,” says the NYT.
A libertarian might agree that the feds should not base any policies based on one’s marital status. However, it’s not that the homo-married should obtain access to federal goodies, but that the normally married should lose all such access.