Stand To Reason podcast of October 3, 2011 (available by clicking on the date in preceding link)
Greg Koukl's discussion with a caller on this issue begins at 2:15:45 and continues to 2:33:02. It is an especially clear and concise response. In this exchange, Greg does not address the biblical or moral aspects of the question -- there are other places to obtain that kind of information. He only speaks to the secular, legal case against same-sex "marriage." I think it is something anyone who is serious about being able to discuss the issue should hear. Check it out if you're interested.