OK, so he has objectionable views on incest. But can we really treat those as a proxy for his views on policy? Non-incest-based policy, I mean.  What if, as a condition of being elected, he promises never to touch the incest laws (and we can hold him to that), but as an accountant, he promises to reform the tax code in a desirable way?

You have to be grateful for a political environment that dishes up real-life examples of a kind you couldn’t have invented in a thought-experiment.