^In the example I mentioned, the woman’s co-parent is the biological father. They had two daughters before splitting up. Her life-partner was the man she started seeing when her daughters were in elementary school (or so she thought, but they broke up recently after several years together).
I know another woman who got pregnant, decided she wanted neither an abortion nor any further romantic connection with the father, and worked out custody and support arrangements with the father and his family, who have played an active role in the daughter’s life. This woman married someone else when her daughter was in middle school.
Neither of these women set out to find a co-parent who wasn’t a life partner, it just sort of turned out that way.
I’m not endorsing either of these arrangements, but ones like the first seem to be increasingly common. It may not be anybody’s ideal, but if we factor out couples who have strong religious views about divorce, it may be getting close to being the de facto norm.
I wonder what sort of expectations the children in these arrangements have (or should have) for their own future relationships. These arrangements seem less than ideal for the kids, but better than the situations of other parents I have known who effectively have no co-parent at all and sometimes no life-partner either.