This article by Robert Patterson argues that traditional marriage is the most economically productive family arrangement because of the sharp division of labor which exists between the man and woman: The man works mostly to be productive outside the home and the woman works almost exclusively within it.  This balances the need to produce economically for the good of the family unit with the need to propogate and nurture which will result in future production and economic advantage for the society. 

This in contrast to homosexual arrangements or even many heterosexual unions in which there is no such distinction. 

Makes sense to me, though this seems more theoretical than based upon empirical evidence.  Of course, this theory supports the Biblical ideal.

In the same article, the author cites some statistics about the state of the traditional family in Scandinavia, where, it seems, more than half of births are to unwed mothers.  If the above theory is true, then I would expect such countries to be poor, yet when I look at per capita GDP it seems as if most Scandinavian countries are in the top 10%.
 
It reminds me of another article I read sometime back which refers to the peacefulness and happiness of these Scandinavian countries.  This was unexpected because I have been programmed to view socialism as an unsustainable, unjust societal structure.  Perhaps a level of socialism can in fact produce a functioning, prosperous, and happy society.