Shows you that people who want to ban same sex marriage because "the Bible strictly forbids it", are picking and choosing what they want to enforce. The Bible has plenty of other things to say about marriages, which Christians are choosing to ignore.
If so many other rules about marriages that are in the Bible have since gone out of practicality, then why not all of them?
If they could come up with a non-religious excuse, then we can have a discussion, but until then, they're just in the wrong.
http://www.whitehouse.org/dof/marriage.asp