This isnt about teaching people about sex vs.nudity, nor is it about American sexual, cultural, and societal norms, (which vary widely). It's to establish the crazy, no-holds barred psychotic character of a very twisted mother, who abandons her family, finds another woman in "her place", then uses whatever tactics to manipulate her sons & get rid of woman #2.
What woman in her right mind would switch from total, unexplained abandonment to instant intimacy? (Crudely, she may or may not realize her nudity may be shocking to her son, but it's not about her son's feelings: it's all about HER - the total narcissist). She does this to rattle, to unbalance & to arouse her son,not for any sexual thrill or act, but to quickly break down his barriers & gain control.
Children need to understand the concept of personal space, of appropriate and inappropriate behaviour, in the context of whatever norm is decided by parents and the immediate society. There is no history, here, to create that context. Kes is confused, probably knows something isn't "right", may be embarrassed, but the point is he is like someone who is asked to get into a shark tank with a "harmless" shark without any knowledge of shark behaviour, without knowing what actions might trigger a shark attack, yet being told he must "love" the shark. He might not even know how to swim, yet he knows that in the water, the shark rules. He has little power. THAT's the point.