Disadvantages of same-sex marriage

Over at Something Awful, Lowtax has a post on gay marriages, and offers the following possible disadvantages to SSM:

1) According to many religious people, God does not approve of two homosexuals marrying each other. These people also believe that God honestly wants them to own a lot of firearms, have gigantic hair, and vote for George W. "H" Bush Jr. III.

2) The legalization of gay marriage leads to a very slippery slope. Once the United States Court approves it, what's to stop people from legalizing marriages between a pet cat and a pet dog? Or a pirate and a ghost? Or a Scientologist and a human being?

3) I saw a movie where two people were married and one of them killed the other one, so this might be unfair for homosexuals who don't want to be murdered, which I assume is a good majority of them.