The current system has a lot of costs and drawbacks. Using the marriage as the norm has caused a lot of issue here in Sweden and the system is currently being tweaked every year to account for a changing culture. 1/3 of couples with children live as cohabitation rather than registered/marriage. Family law has to keep up. Same-sex families with children is an other area which is growing and the law has seen updated several times the last 10 years.
Rather than having laws getting tweaked and warped each year to account for a changing culture, maybe it is time to address it from a different perspective. If a child treat the adults around them as their parents, the adult treat the child as their child, and social environment around them treat the adults as the parents of the child, then why not also have the law treat them as the parents of the child? Outside of medicine does the DNA then really matter?
It would be very interesting if we defined the concept of parents as being the adults with closest social link with the child. The idea of child support as being something imposed would then go away, as child support is generally only contested in the case when the adult have no social link with the child and thus no involvement beyond paying the bill.
It would also make same-sex relations identical to other relations as the law would not care about which adults have closest social link with the child. Marriage vs Cohabitation would also be a area which the law could ignore, and the concept of society taking care of children in need would become natural as there will always be two adults that is the closest social link, even if some of those are paid by the state.