I translate 'marriage' to 'mature monogamous partnership' and I think scientific research shows that people who have a happy long-term partnership live longer than lonely single individuals. Perhaps especially true if children are part of that partnership.

I don't think simply being married equates to happiness though. The institution is probably irrelevant when held up to the quality of the actual relationship.

Correct, it's having a partner that has an impact on our wellbeing. Also, last time I checked psychology textbooks having children had a neutral effect.