Example.
Y "I want everyone to stop buying kittens, and buy puppies instead"
X "I happen to believe that kittens are much nicer than puppies, and I choose to support kittens"
Y "Oh, I suppose you like seeing puppies suffering then? You're one of those people who burns puppies alive and then eats them in a stew, aren't you? You disgust me. You're a disgusting human being. People who are cruel to puppies are often cruel to children - I'm glad I'm not your child."
What Y has done there is set up a Straw Man. Y has decided, in his/her own head, what sort of person X is, and has commented upon this effigy of X, instead of what X has actually said.
I'm seeing a lot of this going on in AIBU and it's lazy debate.