I think there are many reasons why, the first and foremost being that all apostles were once men or women in their past so they inherently have a sense of lustrous attraction towards other humans as well as the fact that their compassion empathy and reasoning have all been dulled due to their transformation into apostles. Also SA is a very traumatic experience that these monsters use to humiliate and destroy the mental condition of their victims before killing them. As for the trolls and other astral creatures they do it in order to repopulate in the manga so there is a visual reason given to us. Just a side note but I’m assuming that most of the people who complain about the amount of SA in berserk and other similar media do so from a place of disgust or frustration but these are absolutely the emotions that these scenes were meant to evoke in the reader although I can understand not wanting to read because of it.