At the time of writing this, it's been at least 24 hours since I first listened to the recording and learned about this occurrence. I've been to work, read though the comments of the forum, and just had a bit of a think.
After reading his text messages and listening to the recording a couple of times, I'm sure that these sexual acts did not start off as consensual. This whole situation reeks of Chris's antics with women. With few other women in his life, who knows how long Chris has been pestering or begging his mother for sexual favours. Since Bob died, he's always been uncomfortably physical with his mother, the same way he has always been with women he wanted to have sex with. Maybe she gave in, maybe he started and she didn't say stop, or he didn't listen.
Megan was right. Before any of us knew of him, Megan was absolutely spot on about his capacity to be a rapist.