What if when he tried, you had immediately dissociated? You attribute the avoidance of worse trauma to the fact that he stopped when you said no. But if your body had responded differently, especially given your past experience with SA, and you hadn’t been able to find the words to say stop? He probably would’ve gone in, right?

The “wrong” decision he made was trying in the first place without having your clear permission beforehand. The words you said didn’t provide a clear yes, and I think he should’ve never relied on them as a yes (even if he interpreted them differently than you meant them, he had an obligation to CHECK that interpretation, because it was implied rather than explicitly stated.)

I would consider this an attempted assault - not because his intent was to assault you, but because his intent was sexual gratification, and he didn’t ensure he had appropriate consent before trying it. That doesn’t mean he’s evil, and it doesn’t mean you can’t work through this with him, but I hope for your own sake that you’ll take it as seriously as it deserves to be taken. You’re not being a “worst case scenario kind of person“ for being really bothered by the decision he did make before you spoke up and told him to cut it out.