There are two situations in which humans are reduced to their most primal, animalistic state.
One is during orgasm and the other is during defecation.
Think about it. What are the two most embarrassing things you can imagine someone catching you in the middle of? When are you at a more physically vulnerable state? You get so caught up in these actions that there is rarely anything else on your mind while they are taking place. Once you start, you can’t stop, and after that point of no return, it’s all just pure mechanical reflex. It’s not uncommon to experience a loss of muscle control (especially in the face) as well as involuntary verbal emissions. Never does a human being resemble a monkey more than during one of these instants. Nothing is more dehumanizing.
And yet it is completely natural.
Brief sidenote: I suppose childbirth might be another good candidate for the list, but as it stands, I’ve only had first-hand knowledge of the two aforementioned activities.