The End of Marriage in Scandinavia makes a very conservative case against gay marriage, seeing it as a cause, not just an effect, of liberalized attitudes towards marriage:
Since liberalizing divorce in the first decades of the twentieth century, the Nordic countries have been the leading edge of marital change. Drawing on the Swedish experience, Kathleen Kiernan, the British demographer, uses a four-stage model by which to gauge a country’s movement toward Swedish levels of out-of-wedlock births.In stage one, cohabitation is seen as a deviant or avant-garde practice, and the vast majority of the population produces children within marriage. Italy is at this first stage. In the second stage, cohabitation serves as a testing period before marriage, and is generally a childless phase. Bracketing the problem of underclass single parenthood, America is largely at this second stage. In stage three, cohabitation becomes increasingly acceptable, and parenting is no longer automatically associated with marriage. Norway was at this third stage, but with recent demographic and legal changes has entered stage four. In the fourth stage (Sweden and Denmark), marriage and cohabitation become practically indistinguishable, with many, perhaps even most, children born and raised outside of marriage. According to Kiernan, these stages may vary in duration, yet once a country has reached a stage, return to an earlier phase is unlikely. (She offers no examples of stage reversal.) Yet once a stage has been reached, earlier phases coexist.
The forces pushing nations toward the Nordic model are almost universal. True, by preserving legal distinctions between marriage and cohabitation, reining in the welfare state, and preserving at least some traditional values, a given country might forestall or prevent the normalization of nonmarital parenthood.