> We could agree that marriage, everywhere in the Western world, is declining in popularity, and non-marital cohabitation rising. This is especially true of Europe.

In my experience, many young couples in western Europe don't see marriage as signifying that a relationship is stabler. It's something you just do after you've been living together for a while since it simplifies some bureaucracy (especially if moving outside the EU) and it's a good reason to have a party. In many countries being in a domestic partnership is very similar to being married from a legal perspective.