Just verbal aggression, nothing physical or anything like that! He gets very upset and essentially yells at me, tells me I don't understand (the phrase "it's alright for you, you can fuck whoever you want" has been used at least once.)
I don't know why. I think it's partly a self-esteem/ego thing, partly hanging out in communities where (it is perceived that) everyone is shagging everyone else, and partly what he's been used to over so many years of being poly and having lots of new people all the time. Bit of a system shock, so to speak. From stories he tells me, it seems that these issues go right back to when he was a teenager.