Let's suppose hypothetically, for the sake of argument, that numerous careful, politically neutral scientific studies have been conducted, and they reveal that children who grow up in two-parent homes have significantly better outcomes -- fewer suffer abuse and neglect; on average they get better grades; they have more education and get better jobs; they live longer, healthier and happier lives.

Let's suppose these studies correctly controlled for all conceivable other variables like race, income level, education, profession, etc.

Don't we owe it to those children to make sure that as many as possible are born into two-parent families? Isn't a financial incentive to marry a great way to balance society's interest in maximizing the well-being of its future citizens in this way, while still allowing its citizens who desire alternative lifestyles the freedom to do so?

I have no idea whether real studies have been conducted that are similar to my hypothetical ones. But based on personal experience and anecdotal evidence, and without knowledge of scientific evidence to the contrary, it seems reasonable to me to assume that heterosexual two-parent families generally have better outcomes.