It's not your fault. It's not that your no isn't strong enough, it's that the guys aren't decent enough to listen to them. When someone says no, it means no, regardless of how they say it or whether they continue to or not. He coerced you into sex, and that is a form of rape. He shouldn't have pressured you. I'm sorry this has happened to you, and I hope it doesn't happen again. Unfortunately, people do need to look out for themselves despite it not being their responsibility to not be raped, so maybe consider getting pepper spray or something.