In other words, by calling them "YND" you're saying that they fit into the majority of the population, who are the deviant, below-decent, whorish, uncaring, ignorant, and selfish people.
In even more blatant/urban terms, "YND" translates into saying that person is just another dumb mother-fucker with which the world is full of already, thanks for adding another one to it, you pissant.
2) Talking to your date: "Wow guy, I thought you were a decent person but YND" which translates to "I thought you were a man, but your just an ordinary douchebag/cheater/etc"
3) Talking with a friend/ex-friend: "I saw you and believed that you were a good person, but after this I can see YND" which translates to "You were my friend for a reason, but you weren't really. You put on a facade and you're just another backstabber/whore/slut/racist/bastard/etc"