Marriage is good if you want kids. Otherwise, there's really no point to it and most people would be happier and better off single. It's also why a lot of married people divorce after their kids are grown.
One study that summarizes previous studies is below, but there are literally hundreds of others.
The Effects of Marriage on Health: A Synthesis of Recent Research Evidence. Research Brief (https://aspe.hhs.gov/reports/effects-marriage-health-synthes... Note: an official US government web site.)
"Married people are generally healthier than unmarried people, as measured by numerous health outcomes.(1) To investigate the complex relationship between marriage and health, this review scrutinizes recent research, focusing on studies that use rigorous statistical methods to examine whether marriage is a cause of these better health outcomes."