Like it or not, a stereotype's existence is based on common denominators.
If you're not a toxic Riven main, then this post shouldn't apply to you. If you really weren't a toxic person, you wouldn't get so defensive when people badmouth toxic players.
I play a lot of Vayne and I still enjoy toxic Vayne memes. Why? Because they're jokes. And they're jokes aimed at the actual toxic players, not me. I'm not about to take offense when someone calls Vayne mains toxic because a lot of them are, that's just the reality of it. Any of these "outplay" champs breed a player base of toxicity who can't handle the realization that they're not as good as they think they are and they just lose it.
If this is not you, then good for you, take joke for what it is and go somewhere else. If this is you, then feel free to be offended, because you probably should be. Honestly if you're a toxic player, I wish nothing but harassment and flame for your entire existence on League until you reform.