> "The biggest problem from my PoV is that people seem like they are always absolutely sure that the contemporary understanding of morals is actually correct, unlike the primitive beliefs of the past, and that there's nothing left to challenge."

If they claim to be on the left, point out to them that that type of worldview is actually a form of conservatism. Quoting from Wikipedia, "Traditionalist conservatism places a strong emphasis on the notions of custom, convention, and tradition." and note that "customs [and] convention" is the same as the "social norms" argument trotted out by the modern left to defend their activities.