Casca was not raped by the apostles. There isn't actually much ambiguity on the matter. The sequence of events in the Eclipse (with tension building up as an apostle gets ready to do the deed) wouldn't make sense otherwise. And by the way, the monsters in this episode aren't so much representations of the apostles as they are representations of the male genitalia. Which is what Casca is physiologically revulsed by... Because she was raped by Femto.