Since the U.S. Supreme Court is hearing arguments today and tomorrow on two cases dealing with so-called same-sex marriage, here is a good video on this topic:
I found this video at the Aggie Catholics blog. I recommend their post on the subject, Why Same-Sex Marriage Is A Bad Idea.
For further reading, I also recommend USA Today's interview with Archbishop Salvatore Cordileone on the topic of redefining marriage. It is one of the best short articles I have read on this subject, particularly in the mainstream press.