Hamster-Bamster, I doubt that the biological mother is just a stranger to him. He was adopted. He probably had preconceived ideas about her before they met. He may have felt abandoned and wanted to punish her. OP did say that she had the impression that he was using his mother sexually as punishment for abandoning him and was rougher with her than he would be with OP. I think this line of thought should be given more credit. From what OP has said, the mother may not just be a biological parent to her bf but a mother who gave him up with all the emotional and psychological entanglements that have accumulated over his 40+ years of life. I don't think she's just a biological parent to him. Would he have this kind of relationship with a random woman he met on the street? Probably not. The facts that she is his biological mother and someone who gave him up for adoption mean something to him. That is what makes the relationship psychologically unhealthy and incestuous.

>>... and in the upper classes of Ancient Egypt incest was common, so there is at least some case for looking at this issue with sober, neutral, calm eyes and not just reacting out of a vague "this is an ultimate taboo" place. <<

Incest being common in the upper class did not make it good. Obviously that practice fell out of favor for a reason.