Unicorns are evil
Think about, in most stories, only virgins can ride unicorns, right? So, this discourages reproduction-because only way little girls (or boys-I'm not sure how this works, really) can ride-and keep on riding this majestic creatures is to never have kids therefore reducing the population of mankind
Unicorns are clearly demonic creatures from the depths of hell! Or agents of an alien conquer! Or the ...space devil, I guess.
I've put too much thought into this