Why do some believe that legalizing gay marriage would lead to marriage to animals and relatives?

After all, gay marriage would take place between two consenting adults, and a dog, cat, or horse would never be able to give its consent. Also, there are laws on the books against marrying relatives, thanks to concerns about inbreeding. Unlike incest, at least gay marriage wouldn’t lead to birth defects!

It’s just the old slippery slope nonsense that never comes to pass. The same things were heard back when people objected to interracial marriage. Can you imagine being so racist that you would say if whites married blacks they would soon want to marry animals? It was said back then, and by my racist great-grandfather by all family accounts.

All this b.s. is proven to be nonsense if we look at Massachusetts. Gays have been getting married there for over 5 years. No one wanting to marry their animals, or their brother or sister, and there has been no decline in the number of heterosexuals applying for marriage certificates (so much for ruining the ’sanctity’ of marriage). Some people just aren’t too bright, and I guess that’s enough to explain this sort of moronic "logic."

EDIT:
geyamala:
Freud was long ago discounted as to many of his theories – especially regarding homosexuality. Maybe you should refer to modern psychology since these are modern times that have seen advances in psychological understanding since Freud. The APA, in the 1970’s, removed homosexuality from the list of mental illnesses, concurring that it is a legitimate sexual orientation. Keeping up with the times helps considerably, and citing old discounted theories does nothing to help your position – it only hurts it.