The underlying notion of the study is found in it's attempt, according to the author to note, “the importance of common biological parentage for optimum child well-being.”
Without going into a few of the challenges to the data, not the least of which is the inability to differentiate between merely cohabitating couples and married ones, the data that he suggests doubles emotional problems comparatively doesn't do that in relation to single parent homes. Why is that important? Because the point of this study is to promote or support legislation aimed at denial of rights to homosexual couples, who fare better (even without the marriage input) than single parent homes.
Well, no one (including the author) is suggesting we deny single parents the right to keep or adopt children.
At any rate, let the exams commence. How good the science and debatable the conclusions may be its more to consider.