What's with the phrase about torturing infants for fun? A tad morbid, don't you think? Well, it's supposed to be an indisputable moral truth. If a system of morality doesn't condemn torturing infants for fun, we can safely discard it. Of course it's hard to find someone who claims infant torture is a morally good or even a morally permissible pastime. So why bring it up? Because it can be modified slightly for use in real-world disputes:
Torturing infants for fun is objectively wrong.The problem with this thought experiment is that we read it with our own "human beliefs and desires" intact. Maybe no one in the imaginary world has a problem with torture-for-fun, but we do. It's like taking our own microbes into a sterile environment, running a microbe detector, and *surprise* there are microbes present!
"[M]oral truths are objective, in the sense that they are in a certain way independent of human beliefs and desires. It is wrong to torture people for the fun of it, and would remain wrong even if most or all of the world's population came to believe that this behavior is perfectly acceptable, and indeed came to desire that it be much more widely practiced."1
When arguing that moral truths exist independently of human emotions — as Plantinga tries to do in the quote above — it's important not to use emotionally-charged examples. Suppose some form of moral anti-realism is true, i.e. moral truths are at least partly dependent on our "human beliefs and desires." We would expect emotionally-charged scenarios to produce especially strong moral judgments, not apathy! This is why I roll my eyes at claims that people can't "live out" skepticism of moral realism because we still care about things.
We can even turn this around on moral realism. Truths which hold completely independently of how we feel about things are not necessarily going to excite our emotions. Why would moral certainty and strong emotions tend to show up together? Anti-realism has a simple answer: morality is (at least partly) based on emotions. Realism needs a more elaborate story.
1. Plantinga, A. (2010). Naturalism, theism, obligation and supervenience. Faith and Philosophy, 27(3).