I'm reminded of the book Gay Marriage (http://books.google.com/books?id=kp6rA_wbWy0C&dq=same+sex+marriage+good+for+america&psp=1&source=gbs_summary_s&cad=0) by Jonathan Rauch, which offers what is essentially a conservative argument in favor of same-sex marriage. His thesis is that couples making a commitment and taking responsibility for one another in the way that is both socially signaled and legally implemented by marriage is good for society, and is therefore something which society should expect of people, including people who choose a partner of the same sex.

As he points out, the idea that people who want to live together and (particularly) raise a family ought to get married is slowly eroding, and if the conservatives want to save it, their only hope is to legalize same-sex marriage as soon as possible.