How? Seriously how is gay marriage in any way similar to slavery?
Did he just throw darts at a list of "bad things" and randomly chose one to connect to gay marriage?
"Hmm Gay marriage is the same as Hitler... Nah, Godwin... Gay marriage is the same as Dentists, nope doesn't make sense... Gay marriage is the same as Contraception, hmm this one might do, Gay marriage is the same as slavery, oooh looks like we got a winner!"