Children are about much more than finances.
Thing is, you don't have to be married parents to have custody of children. You can be a single parent (as my wife was, before we married). You can have custody as grandparent, older sibling, other blood relationship, or even adoption by non-blood relations. You can be a married couple and have the mother be a surrogate parent for another couple without maintaining custody.
Heck, between the support both my parents and my wife's parents are providing, my kids are being raised by an extended family for all intents and purposes -- and I think it's even a better arrangement, on the whole than a nuclear family. Neither of my kids would've been able to attend the private school they've attended without grandparents chipping in financial support.
Yes, children about much more than finances. For better or worse, they're about a lot more than marriage, too.