It doesn't really matter whether this is sexual assault or not imo, what matters is that he doesn't respect your boundaries. "No" means no, whether that's a sexual act or saying no to literally anything like whether you want cereal for breakfast. You don't need a reason and you shouldn't have to repeat yourself. His reaction just needs to be "okay I'll stop" like there's nothing else to say when someone says no and the fact that he's getting upset and guilt tripping you over it is a huge red flag.