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 :)
Still, I would argue this:
Lets say your 19 year old daughter wants to introduce her boyfriend of same age into your family.
How would you rate her (and his!) seriousness if
a) they lived in his apartment or
b) they are organizing their wedding.
I would wager b) would be "well-established" much quicker, especially in the wider circles, and thus have more "social resources" sooner, thus giving their potential offspring better chances. Of course, with kids out of the picture, it wouldn't make much difference. But that is exactly my point: Marriage serves having children.
Your "(a) living together" is a much more serious and well established relationship than "planning wedding".
"living together" implies a de-facto relationship with the legal protections that brings. "Planning a wedding" gives none of that.
To your point: legal protection is a very serious "social resource" given potential offspring a benefit.
The data show that married-before-cohabitating situations have more robust relationships for raising children.
In many US families, it's not uncommon to take people you're seriously dating on family vacations, or have them over for holidays, etc.
Some people who choose not to get legally married have a ceremony and/or party.
Some people who get married don't change their name. People who don't get married can change their name if they want to.
- In-laws - you become a member of a new family, new levels of mutual privileges and responsibilities
- Public wedding - Proclamation to both social circles that the new family project now has officially started
- Family name - new Identity
- ...