Confusion

So I’m with my ex again and we picked up our relationship after he broke up with me and we dated other people he’s done a lot of things that’s made me uncomfortable like saving porn models on Instagram and saying it was an accident, and watching porn when I live with him. Once we finished having sex on time and thought it would be funny to bring up the fact that he wanted to have a threesome as a life experience and I had already told him that I didn’t want to bc then I would have to think about him being inside another girl while we were together and he said that it shouldn’t matter bc he has to think abt my ex that I dated after him being inside me. I’m just confused and don’t understand why that even matters I need a dudes view idk maybe it’ll help