One of the most annoying arguments used against gay rights is the "slippery slope" argument. I.e. we give gay people right and next we know men will be marrying their pot plants.

I don't think it's a slippery slope at all. It's not a slippery slope but plain fact warping. There must be a reason why these people overlook one very simple fact: a [gay] person getting married is a capable adult person who can consent and make decisions. A pot plant or a pet duck, however, is not a legally capable consenting adult. Comparing gay people to pot plants is being illogical on purpose.