From the neighbors in the Netherlands: it is interesting to see these local differences as at least in my circles you don't need to get married to become part of the in-laws or other social circles.

Really, most couples I know really only marry when they are already very well established in each-others circles, as sort of a affirmation of their love. Maybe this is also because legally you can achieve the same protections for your children a lot simpler than getting married.

But this doesn't mean the whole of the Netherlands works like that, just my social circles :)