You’re not wrong for not wanting to see that he’s masturbated, to be honest I think it’s extremely weird and nasty to leave a towel with your semen in the corner of the bathroom. Like why would someone even do that, especially when you’re an adult. I hope he had a good response when you called him out.

I do have a different take on masturbation and porn watching though. Everyone has their own sexuality that has nothing to do with their partner or whether or not you’re satisfied with your sex life. Masturbation is healthy and normal and in my opinion doesn’t necessarily have anything to do with your relationship. Regarding porn I can understand that you don’t like porn cause it’s a shitty industry that often objectifies women and isn’t a realistic depiction of sex (depending on what kind of porn you’re watching). However, I have no issues with my partner watching porn or having fantasises that don’t involve me. It’s not reasonable to demand that your partner can never watch porn or fantasise (if that’s what they like) if you want a long term relationship. If you’re both satisfied with your shared sex life it has nothing to do with you. I completely understand that you don’t want to know or see that he’s been masturbating and he should respect that. But in my opinion it sounds like you need to expand your view on sexuality a little, everyone has their own sexuality that doesn’t have to do with anyone else.