Right, women can marry men...but men can't marry men...thus allowing women to do something a man can't do under the law and vise versa.Right, women can marry men and vice versa.
What if the law said that every race can marry anyone of a race that isn't their own, and then said its perfectly acceptable under the EPC because "White person X can marry anyone that's not his race, and Black Person Y can marry anyone that's not his race, so its exactly the same". Or how about "Religious people can marry religious people and Athiests may marry athiests, but they can't marry across that line and its perfectly okay because they both are able to marry the opposite religious preference".
It simply hasn't reached the Supreme Court yet. There have been cases already in the lower courts that have ruled its an issue of Gender under the EPC. That said, simply because something hasn't reached the SCOTUS yet doesn't prove constitutionality.Then I would imagine someone at sometime would have challenged them and brought them to the supreme court to decide.