Here is one reason that marriage is good for society: marriage fosters the virtues of altruism and responsibility. This is not something I would have truly appreciated without having the actual experience of being married. When I was single, I was never required to consider anyone but myself when making decisions about the course of my day or the course of my life. Now that I'm married, there is always someone else to consider (and to involve), not only in the occasional big decisions (choosing jobs, choosing a home), but in daily mundane decisions: picking groceries and planning meals to fit two tastes (and allergies), deciding how late I can stay at work depending on my husband's work schedule, figuring out how to get dinner cooked by 7:30 (because one of us gets cranky if it's much later than that), deciding when to go to bed. In a dozen small ways each day, I'm thinking about someone besides myself. Many things that I do now, I do not for myself but for someone else.
Virtues are cultivated through practice, and by living in a marriage, spouses learn the habits of altruism and responsibility, which over time build character. With the habit of thinking beyond myself to my husband, it becomes that much more natural to think of others. By doing things for my husband, it becomes that much more natural to do things for others. This is not to say that single people can't be altruistic, or that married people can't be selfish. Persons vary. Of course we all generally understand that altruism is a virtue, and we should always strive to think of and do things for others, for the larger society. But the claim of the larger society, of "others" in general, is diffuse and seldom makes specific claims at specific times directed personally. Thus, only the more saintly among us could diligently make a firm habit of altruism just from the diffuse claims of humankind (even with the urging of our pastors and priests). But with the specific "claim" (and opportunity) presented in marriage, more of us can learn to become more altruistic and responsible. Marriage thus fosters these virtues, building character in its participants, which benefits all of society.