I don't think Chris knows what kind of rape he committed.
Considering that Chris' worldview is interpreted through television, like the lead from Being There, Chris believes rape is the obviously horrible kind. The kind you see on a network drama. Some shady stranger approaches a woman from behind, knocks her unconscious, and violently has his way with her. That kind. What he fails to understand is he committed the kind of sexual assault where you take advantage of someone vulnerable and is easy. The drunk girl at a Frat party kind.
And this tracks with Chris. From Megan, Catherine, and, now, his Mom, Chris has never learned "No means no!" He's always been too aggressively nice to women. There's that e-mail exchange with Catherine where, despite her having already broken up with him, Chris still calls her romantic nicknames, like "Honey" or "Sweetie." She keeps telling him to stop it, but Chris refuses because, in his simple mindset, what could possibly be wrong with being too nice? He can't comprehend that's sexual harassment.
So, because (he thinks) he didn't hurt Barb, and he hit "her G-Spot," it's all good and fine and not rape. I can't see why else would Chris tell Bella that Barb said no? I mean, I believe Chris knows incest is wrong (or at least he once did and that belief has eroded over time) and that's why he was hiding Barb's identity from Null, but I don't think Chris realises he raped his mom.
TL;DR Chris doesn't understand consent.