JT Eberhard's blog recently tipped me off to the videos of John Corvino on arguments for same sex marriage (by dismantling arguments against SSM). His voice is soooo soothing, too.
Here's the 1st: The Definition of Marriage
The 2nd: If Gay Marriage, Why Not Polygamy?
The 5th - Is Gay Marriage a Threat to Traditional Marriage? - is also quite good.
There are nine videos in total; they're all (IMO) quite good.