For a more complete case against same-sex marriage than is typically offered, take a look at the recent piece by Sherif Girgis, Robert George, and Ryan Anderson (former Anscombe president, and friends of the group respectively):
http://library.constantcontact.com/download/get/file/GGA%20-%20What%20is%20Marriage.pdf
Once I have a little more time on my hands (read: during break), I’ll look at this a little more closely and parse through some of the arguments as they apply to related issues/look at the responses offered to particular objections.