I don't think this example helps much, because it's not that we don't know what an unpleasant truth feels like. Everyone knows what that feels like. The difference is in what comes next. Do we accept a painful truth or accept a happy lie? To me, rationalists like Sam Harris, and probably most programmers, truth wins over feelings by quite a long shot. A better way to understand the other side, is to realize that for a lot of people this isn't the case.

Here's a better example. If you were being cheated on, but this had no affect on your otherwise perfect relationship, would you want to know? What if it was a one time thing that your partner was extremely regretful about and would never happen again. What if you're in your tenth year of the relationship, the cheating occurred nine years ago, you currently share a mortgage and two children.

I would still answer yes, and you probably did too. But many people don't care about truth that much. And I don't necessarily think they should.