Your responsibility to care for your children isn’t connected to marriage (well, not where I’m from, I don’t know if some states have bizarre laws surrounding this). A lot of the time custody and support decisions arise during divorce for obvious reasons, but they aren’t the same thing. People run off without getting divorced too, people who were never married raise children together.
But I think the question is whether we need to protect society against frivolous divorces by whimsical people. And I think the answer obviously no since those people are extremely few in number (if they exist) and there isn’t a reason to stop them from getting divorces they want anyway.
I used to work for a woman who was still married to a man she hadn’t seen in nearly a decade (and who she didn’t really plan on ever seeing again) and the government wasn’t coming to intervene. Society’s interest in who gets married and how long those marriages last seems pretty minimal to me.
