mike777, on Oct 15 2008, 04:46 PM, said:
I certainly hope one can have a stable family and not be legally married or have an unstable family and be legally married.

Again many kids if not yet most are born out of wedlock. Many kids if not yet most are raised by single parents.
But the presumption is that they would usually be better off being raised by two, loving parents in a stable relationship.
Yes, you can have a stable family when you're not married, but again the presumption is that this is more likely if you've made a powerful declaration of commitment by getting married.
These are all generalizations, but most laws are based on generalizations. For instance, we have drinking and voting ages based on generalizations of when most people are mature enough to participate in these activities, because it's too hard to judge every case individually. We recognize that this may disenfranchise some people, as well as give privileges to some who don't deserve it, but we live with these faults because the alternative would be impractical.
Thousands of years ago, when we lived in tribes of a few hundred people, the chief knew everyone and could make individual judgements about what was appropriate. We didn't need systems of laws then. But when you have societies of millions of people, you need common standards or you end up with anarchy.
Libertarianism is a nice ideal, but it doesn't really work. Society is much too interconnected. You say that you should have a right to decide whether to wear seatbelts, it's your own life. But if you also have insurance, your decision affects actuarial statistics, which affects everyone's premiums.
Let's suppose children raised by single parents are more likely to become delinquent (someone can probably find a study that says this). Juvenile delinquents often commit crimes, which affects everyone else. Thus, society has a stake in reducing single parenthood.
There are probably very few important life decisions that don't have implications on others.