"It is your duty as a citizen to vote", they tell me. Pish-posh is what I say. If I go into a restaurant and they present the special of the day, and it is liver, I will not choose it. If they tell me that the only alternative is spider, I will not choose that. I would rather go hungry than eat either.
So don't vote. To vote for someone who supports objective evil is to choose objective evil. This does not mean that we have to vote for the alternative. Just because I cannot vote for one does not force me to vote for the other. No one can force us to choose.
But then we will leave the choice to those who do vote! What about it? If 10% of the ballots did not include a vote for president, I guarantee that next time someone will try to figure out how to get that 10%. The Christian political act, according to C.S. Lewis, is to convert your neighbor. When we form a contingent of people who will most certainly not vote for someone who wants to have his grandchildren killed, and will not just vote for "the other guy" either, then we will be a powerful force in politics.
But if you are in Minnesota, do vote. Go vote "Yes" on the definition of marriage and write in your Aunt Sally for president.