I feel like the sudden revelation of Barb's "proposition" may have come from some time with a Psychiatrist, and Chris giving them the "Barb started it!" recounting that he's offered up numerous times in the past year. He told the Psych that "Barb made da first moves" and was asked "OK, Do you remember anything before that where Barb made similar suggestions? What was that like?" as a way to get more information out of him, and Chris, notorious for oversharing, told the story of the prostitute and Barb (likely angrily) saying that, which ran through the Autism Filter and came out to Chris as Barb actually meaning it. It probably has bounced around in his head ever since. It was only four years later that he admitted to having dreams of fucking Barb.
I don't think this is going to end up as his "get out of jail free" card, but I would be surprised if it didn't end up in his evaluation as some kind of autistic trigger for what happened last year.