They're asking over on MarriageDebate. Well, if you seperate procreation rights from marriage, then it's hard to see why marriage is the government's business. What is obviously the government's business, though, is the welfare of its citizens. And because the creation of new citizens might happen whenever a man and a woman have intercourse, the government has an obligation to ensure that intercourse is only done by people who have made the proper binding commitments to each other and any citizens they might create. A marriage license is a public license to start doing what might make people, and along with it comes legal responsibilities. I agree with Maggie that there can't be legal responsibilities without the government being involved.
It is foolish to think that sex can be kept private, sex always takes place in public because it always has the potential to become public nine months later. No one suggests that it isn't the government's business to hold fathers and mothers responsible to their children, or even to each other. Perhaps modern child support enforcement, abortion, and paternity testing has made it seem like we can worry about responsibility if and when children arrive, but what about couples who don't have children, even after years of trying? Do they not need some public support and protection? They have no way of knowing if a baby will arrive, and must live their lives with the same responsibility for each other and their future children.
The questions about the word "morality" seem to be afraid to mention the 'f' word - is it the governments business to say it is illegal to fornicate, to have children, unless you are married? Does the mere existence of a fornication law cause people be responsible? I think, in my experience, that it does. I knew, and my girlfriends knew, that it was officially, legally, authoritatively, wrong. Sex was something that people shouldn't have unless they are married. And that was a very good excuse for either one of us to say no. And if we enforced the fornication law a little better (either of us, as well as the government) we might even see a reason to get married.