like pretty much everything in a healthy life, it's a balance of all things. while i'm not entirely into this 'first vs second' mentality, as someone who is 40, married for ~16 years, and also with four kids, I've seen plenty of my peer marriages just lose all inertia cuz the parents overfocus on "kid shyt" and lose their identity a married couple. on the flip side, i've also witnessed plenty of kids get left on the sidelines while mom and dad refuse to grow up and acknowledge that one of the core foundational responsibilities is ensuring a stable home life for their kids.
both lead to pretty bad outcomes IMO. that said, the "strong marriages raise strong kids" is something I can cosign.