Except that the lesser evil takes your vote as explicit support of everything they're doing, and will claim it as a clear mandate, we'd like to stop but our hands are tied the voters wanted this.
And then other way to look at it is that by not voting, you’re saying you’re completely fine with the choices and are willing to allow others to chose for you.
It might be incorrect, because they don't know your motives, but that's the whole point of voting - showing your motives. In a voting democracy abstinence is begrudging consent to either candidate. The two party system forces this reality.