OOOOO, a "political comic." People use the slippery slope argument fallacy all the time when 'debating' about gay marriage. They say "What's next are people going to marry their dogs/goat/lamp?" Why would anyone even bring that up unless they secretly wanted to marry their dog? Are they just trying to gage your reaction to see if you will except their secret love? The difference between two consenting aldults getting married and a person molesting their pet seem pretty apparent to me but maybe I am just not progressive enough.
For the record I am pro-gay marriage, anti-dog marriage but gay-dog marriage is a moral grey area for me.