Marriage laws are also determined by states, but that doesn’t stop federal judges from throwing out state constitutional amendments that define it as between a man and a woman. (see Kalipornia just recently)
Again, federal law (Taft-Hartley Act) expressly authorizes States to be RTW. If there was ever a federal judge who would rule Taft-Hartley to be Unconstitutional, the unions would have gone to him long ago (T-H was passed in 1947).