she couldn't tame a unicorn

Unicorns are most widely thought to be simply horses with horns, but a quick Wikipedia search will inform you that the mythology also states that unicorns can only be tamed by female virgins. Hence, if you say "she couldn't tame a unicorn," you're saying that she's not a virgin.

Guy 1: What do you think of that girl over there?
Guy 2: Her? She couldn't tame a unicorn, no chance in hell.

Guy 1: I concur. I bet she's never even been within a thousand miles of a unicorn.
Guy 2: She alone is the reason why unicorns don't exist in this world; they couldn't possibly occupy any place that is far enough from her without spoiling their innocence and purity
Guy 1: Hmm ... so that's why unicorns don't exist ... interesting theory.