Everyone with experience as a humanities professor is aware of the "it's all relative" mantra of college freshmen. Justin McBrayer believes that their moral relativism in particular -- the fact that they don't believe in "moral facts" -- is attributable to lapses in their K-12 public schooling.
I doubt it.
Like the philosopher Michael Sandel, I see such relativism as an unintended consequence of liberalism; not, like Sandel, as a logical consequence, but rather as a function of the natural tendency of the liberal ethos, and the open mindedness it requires, to decompose into their opposites. Attempts to embody a skeptical suspension of judgment at a political and cultural level (separation of church and state, freedom of speech, etc.) are easily lost in democratic translation, and transformed culturally into "man is the measure of all things." There is the further factor here that how we can say that normative claims are true or false is intuitively far more puzzling (and traditionally has been the greater epistemological challenge in philosophy) than grounding non-normative claims. So it should be no surprise that moral claims seem less epistemologically secure to students than empirical and scientific claims: many philosophers believe the same thing.
"It's all relative" is just the way that college kids try and fail to live up to their egalitarian (and proto-skeptical) impulses. As exasperating as it can be to philosophers, it's also salvageable: not by demanding that students yield unquestioningly to the dogma that there are such things as moral facts (because it is not obvious that there are); but by leading them to see that moral relativism is just as philosophically problematic as moral realism.
McBrayer conflates moral anti-realism and moral relativism, and I didn't attempt to untangle this distinction in this post. Suffice it to say it's moral relativism I'm referring to as standing in opposition to open-mindedness, which I connect with a certain kind of skepticism ("zetetic skepticism," associated with Pyrrhonism) involving the suspension of judgment (and what Keats called "negative capability"). That's because the outcome of relativism is that any assertion I like will do, leading to a kind of lazy dogmatism. Moral anti-realism, however is a coherent philosophical position that I allude to toward the end of the first paragraph. The point is that there is a psychological relationship between moral relativism and positions like liberalism, zetetic skepticism, and moral anti-realism: moral relativism tends to get conflated with them. This does not mean I'm endorsing this conflation. Relativism is actually a dangerous foe to liberalism. So it's an unfortunate irony that liberal impulses so easily degrade into relativistic ones.