Rape wasn't seen as bad to the barbarians who did it to women who weren't part of their tribe, yes. But obviously it was bad to those who were dealt pain from that very act.
The idea of it being bad isn't just because it is inferred from morals or social landscape; it's that pain was dealt, thus it is regarded as bad to those who were dealt that pain.
Morals and ethics aside, you can't take into account the reasons behind rape yet omit the direct, physical consequences it has on the victims.