There are only a limited number of studies, that are several years old, that studied differences between heterosexual and homosexual relationships and found substantial differences between incidence of monogamy and the longevity of relationships. I suspect more haven't been done because the organizations that fund these kinds of studies tend to be highly political, and wouldn't fund studies that work against their interests.
I would personally argue that a marriage is about creating the stable family structure for having children. Monogamy within this relationship is driven to ensure paternity and to protect the children from being abandoned by their father. The life-long vows and social stigma about breaking up this kind of relationship was to ensure that the shared resources of both parents (and their families) are available for the children. The entire arrangement was in preparation of children ...
Being that children raised in single parent households are 9 times as likely as married households to be involved in crime, the poverty rate of single parent households is 6 times as high as married households, incidence of childhood obesity are higher in single parent households (26% vs 22%), and most studies demonstrate negative results of single parent households, while children can be raised outside of marriage it is obvious that they do best in stable dual-parent household; most often represented by married parents.
You're demonstrating correlation, not causation. The fact is people in poverty, involved in crime and generally on the wrong end of social demographics are more likely to be single parents. It's not the fact that they're single parents that are causing the kids to turn out bad, it's all the other factors that are causing them to become single parents.