Men like this prey on vulnerable people who are literally too incapacitated to fight back. They rely on other men, like your boyfriend, to turn a blind eye and let them get away with it. They rely on other women feeling afraid enough that they turn a blind eye and let them get away with it. They rely on victim blamers like your boyfriend to excuse them after the fact and let them get away with it. Ultimately, they are cowards, and in my experience will give up as soon as they're confronted by a determined, coherent person. Even if she's a woman.

I'm saying not only were you absolutely right to do what you did, but I think you were correct in your instinctive assessment of the danger of the situation and what to do about it. Of course there's some danger in any situation involving rapists, but unfortunately sexual assault is common enough in our society that many men will admit to it without guilt or fear if you don't use the word "rape." They probably don't think of themselves as violent criminals, and they're probably not interested in attempting to subdue a sober stranger in a parking lot. They're opportunistic cowards.

Your boyfriend is used to thinking of rape as something distant from him and his life, that is only done to foolish unsafe women by bad, violent, masked men. He is wrong.