Counterpoint: I'm young, but I still measure my core friendships in decades ...
I'm not going to settle for a life where 'romantic partner' is not a core friendship, one which I can rely on in the same way as my lifelong friends and close family.
I think that a lot of people feel the same way.
To me, that's more persuasive even than the 'think of the children argument', and much less likely to change.
The 'think of the children' argument is pretty strong here. We all know the stats: kids from single-parent families tend to grow up in more negative environments, tend towards worse outcomes.
But maybe society can solve that problem! For instance, if a kid's parents are divorced, but are constantly in caring relationships, maybe society can evolve to a point where those kids aren't disadvantaged, and where they have a relatively stable upbringing.
But that won't change my mind about 'romantic partner' belonging in the category of 'secure, long-term friendships.'