And at the root of this is the very idea of marriage. Marriage is more than just having kids and raising a family. This Darwinian view of marriage focusses on ensuring the success of the next generation. There is more to it than that. Many marriages are childless.
Many marriages have kids but then the kids move out, leaving the parents as empty nesters.
A spouse may die, and the remaining spouse may remarry. Marriages move and grow through time, because each person in the marriage is undergoing changes and movement through time. The older one gets, the more one appreciates the strength that comes from having a companion to share the ups and downs of life with. Those kind of issues are completely different from those facing couples who are just starting out or who are in the middle of raising families, the issues that are behind most of the laws set forth in the religious and civil arenas. At some point, though, the simple human need to have a companion, someone to share life's ups and downs, someone to grow old with, that need has its own validity.