Gay marriage strengthens marriages - the statistics are in

Ed Brayton at Dispatches from the Culture Wars has a summation of what is actually a number of studies. There are several studies from countries where same-sex marriages are allowed, they point to the same conclusion and it's not what the opponents of same-sex marriages say.

For a more in-depth look checkout the book by Eskridge and Spedale called "Gay Marriage: For Better or for Worse? What We've Learned from the Evidence."