Whilst it's a topic that can get very personal (which is why invariably political discussions ultimately get heated very quickly, as this thread has shown), I think the point here essentially boils down to pointing out what not voting means.

In a system where you have one side doing their best to amplify the weight votes for them have (whether that be through gerrymandering, the electoral college, etc), not voting just increases that amplification. So whilst on paper not voting demonstrates your lack of confidence in either side (and let's be honest, without some real changes it's a two horse race), in reality that decision is primarily (not exclusively, but primarily) benefiting one side of politics.

If you're making the decision to not vote with full knowledge of what that means, realistically I think that's all someone can ask without getting into a discussion about trying to change your mind about that decision.