I think there are always outliers to every theory but I would say that cohabitation before marriage is probably harmfun. I think one should probably make a committment and live that committment for awhile before getting the fruits of that committment. Now does that mean that people shouldn't be able to make babies before getting married? No I don't think so. I think these studies are interesting to show what is happening or what has happened.
In my opinion I think cohabitation before marriage is a big reason divorce is common. Cohabitation is probably where a lot of these problems begin. I think when people cohabitate before marriage they tend to miss some vital steps that help ensure their marriage will last. Of course, these are all different for different relationships.
But again I'm not sure the op was trying to say that the government should force people to marry before they live together but was just pointing out the study. You can certainly disagree but lets try and be respectful.