Al and Peg are a typical American couple who have cohabitated for many years. If they later marry each other, they are likely to be less happy than other married couples who did not cohabitate.
This can be explained by the daily routine in combination with marriage cliches and responsibilities that gradually erase initial stage of romance and passion that married couples tend to share in the beginning of their life together. If a couple that did not cohabitate starts a married life, there is still space for romance which is the essential element that keeps the family together.