Whenever I assert that the central purpose of marriage is to protect children, someone usually objects that not all marriages are blessed with children. How, they ask, can an institution be about something that is not always present?

This is a fair question, to which there is a good answer. Consider the rules of the road. American drivers know that when they get into their automobiles they are required to drive on the right side of the street. They also understand that without this rule, the gridlock would be so bad that no one could ever get anywhere.

Nonetheless, there are times we have the roads exclusively to ourselves. At four o’clock in the morning we might be the only vehicle for miles around. Even so, we are required to keep to the proper lane. And we do.

But why do we? Why don’t we simply throw caution to the wind and aim our machines anywhere we desire? If it sounds silly that we do not, there is actually a very good reason that we don’t — and it does not involve the fear that we may get a ticket.

The reason is habit. The need to keep to the appropriate side of the road is so compelling that doing so must be reflexive. It needs to be something we do not think about, but take for granted. Only in this way can we ensure that people — and not just us — do what is necessary most of the time.

This same social strategy applies to marriage. Some social institutions are so deeply ingrained in our hearts and minds that they seem natural. They have to be because there are times when we might be tempted to violate them that are so egregious we must prevent this from occurring.

Mind you, social rules are broken with tedious regularity. Every society has an injunction against murder, nonetheless murders occur everywhere. Yet there would be far more murders if we did not take these proscriptions seriously. If we ignored them, then whenever we felt insulted there might be blood on the floor.

It is the same with marriage. Marriages are supposed to be lifelong commitments. When people agree to wed, they publically vow to remain together until death does them part. Not all do, but the fact that they take these promises seriously enhances the prospects that they will.

This is of particular importance to children because the benefit of having two parents is so great. A mother and father dedicated to remaining a couple are likely to be dedicated to protecting their offspring. There are exceptions, but emotional loyalties improve the odds.

These attitudes are embedded in people when they are very young. They derive in part from the importance that society attaches to marriage. The reason that nuptial ceremonies are public affairs is so that the community can add external pressures to personal desires.

If this is so, then tampering with time-honored marital traditions may be more dangerous than many people suppose. If traditional marriages strengthen the bonds between individuals, then scoffing at this custom may loosen attachments upon which we all rely — especially children.

Which brings me to the subject of gay marriage. Gay marriages may be a good idea — but then again they may not. If in imposing this recent innovation people become convinced that marriage is arbitrary, the sense of sanctity with which it has been surrounded may be reduced.

Then where will we be? Will we feel free to drive on whichever side of the road we please? Will we decide that multiple spouses are OK? Or that cohabitation is just as good as the old-fashioned kind of marriage?

But if we do: What about the children? Will we also decide that a commitment to them is arbitrary? I hope not, because if this transpires the amount of personal unhappiness will be staggering.

Melvyn L. Fein Ph.D. is professor of Sociology at Kennesaw State University