I believe the reasoning would be that it's good for kids. Statistically it is - relative to divorced or unmarried parents.
If the new norm were flexible open relationships however, that wouldn't necessarily correlate with worse outcomes for children. I think we just don't know.
Perhaps there's a heavy correlation confusion here -- I expect it's more like "If you're the type of person who would stay in a stable marriage, then your kids will do better than those who aren't, but you don't actually have to get married to confer that effect, it's just about what type of person you are"?
If the new norm were flexible open relationships however, that wouldn't necessarily correlate with worse outcomes for children. I think we just don't know.