Caska gets raped by griffith, Guts gets raped, Farnese is almost raped by a horse, and the trolls rape everyone. I find the biggest reason so much of it is there is to show the common evils between Apostles/Demons and humans. For Casca she is a woman warrior in a world dominated by men, all of whom would sadly very much violate her, but she would overcome it always, she'd fight off those monsters and their terrible attempts which shows she has.