Still, I don't have a very hard time imagining living alone (even though I'm in what we have just decided to consider "marriage" and we have kids), that seems a perfectly fine life to me, which would have also a center, which marriage would not be, even with this definition.

Even in my situation, I would argue that the kids are more in the center than our relationship, as our parent-kids relationship is inalienable and forever, unlike our relationship with each other, which will last only while we are both happy about it.