1. It's Not Your Duty. Whether or not you agree that U.S. military policy is about protecting democracies, in the U.S. you have both the right to exercise your vote, and your right not to. A small number of democracies, from tiny Singapore to systems as large as Australia and Brazil, require most or all eligible citizens to vote. Maybe you genuinely feel other citizens will make the right choice for you, and you'll stand behind any outcome, not because you're apathetic, but because your true passions are tied up in birdwatching or poetry. Or in defending democracies in Halo 4. Just don't complain about anything. Ever.
2. You Disagree. Abstention has been used in numerous moments of history as a protest mechanism, and for those who object to U.S. policy, whether foreign or domestic, burning the ballot could arguably be a legitimate form of voicing political opinion — your way of voting. The obvious problem with this argument is that, unless a social movement has publicly organized an abstention campaign to distinguish abstaining from apathy (one good way to do this is to urge protesters go to the polls and cast a blank or invalid ballot), staying home is, for all intents and purposes, indistinguishable.
3. You're An Anarchist. Q: How many voters does it take to change a lightbulb? A: None, because voters can't change anything. The "mainstream" anarchist philosophy eschews electoral participation. It's a form of collusion, legitimating the system. It intrinsically entails oppression of minority groups by a majority with the means to, well, create a majority. But beware of armchair anarchists, hiding apathy behind a thin veneer of empty rhetoric. Unless you've got Mikhail Bakunin, Emma Goldman, and Noam Chomsky in your library, or a balaclava in your wardrobe and stand prepared to throw your body like a wrench in the works, you're probably just a tool.
4. You're A Libertarian. Even though the Libertarian Party is one of the two minor parties on the Bexar ballot this cycle (the other being the Greens), some hardcore libertarians believe that "alls ya gotta do in this country is live, die, and pay taxes." Maybe not even the latter, if you live off-grid in a trailer outside of Terlingua. What's the difference between libertarians and anarchists? No central authority on the matter, of course, but crude rule of thumb: libertarians embrace private property, anarchists don't.
5. Your Religion Prohibits It. When asked about Obama's candidacy on the sidelines of the 2008 Wimbledon, Serena Williams replied, "I'm a Jehovah's Witness, so I don't get involved in politics. We stay neutral. We don't vote." Several other religions discourage or outright forbid participation in "the earthly world" of politics, including Shakers, Quakers, and the Amish. Who are we to judge?