what makes you think that? It doesn't fly... Take an example, I mean you have two couples living in Paris that are good friends and have decided to get married the same day, one couple is made of two British nationals and the other couple is French, they have rented a nice castle in the French country side where they planned to have a lavish party with their numerous friends and family. The British couple know a very nice Church in the country side in England and they know the Priest there and have made all the arrangements so the two couples can marry the same day, only close family will travel for the ceremony at the Church. Alright. When they come back in the eurostar with the Church document, the French state will one couple is married and the other isn't....? Based on what? their citizenship?, in one instance they will the British couple is married and because the French couple only married at the Church they will tell them that they aren't married, well that doesn't fly with the EU laws.... Both couples are married and where and who did the celebration cannot be used to discriminate one couple regarding their status under French laws. So what we have right now is if an English Priest marry a French couple in an English Church they are married for the French state but if the same couple is married by a French Priest in a French Church('d be the same with a Rav) the French state says they aren't married....
So I think they have a point. It can hold for very long given the French state cannot argue within the European Union that a marriage isn't a marriage for the law because it was only a religious marriage, therefore discriminate  religious marriages based on the citizenship of the Priest or the geographical location of the Church (regardless of the obedience) is to me against the European rules and impose to married couple a fake ceremony at a city hall celebrated by some politician to access rights reserved to married couples can't last much longer....