"Or are the upstanding men the ones who marry in the first place?"
An old mystery. The linked article is about a study that shows there's combined causality: Marriage tends to make men more upstanding citizens and more upstanding male citizens are more likely to find and commit to a mate.
Related mysteries: Should we incentivize marriage and pressure men into marriage in order to make society better for all of us? Must men marry women to get this social improvement or will gay marriage work too? Should women be enthusiastic about performing the function of improving men? Is the restriction of marriage to opposite-sex couples a way to enlist unwilling women in the social enterprise of improving men?
CORRECTION: I'd originally written "same-sex" in that last question.