Do atheists even need an objective moral system?

[Jackalope]

No, we do not - and I'm not certain we could have one even if we wanted one.

As to the rape question - I'd turn that around on the questioner, and pose this questions (asked in the context of a world without an real or imaginary deity): Are you saying that we would be unjustified as a society in prohibiting and punishing rape?

To me, the answer is a resouding no, we would not be unjustified, because we as an empathetic society value our society free from rape. Why? Because we do.