Why didn't you assume that he could be autistic,, may I ask?
He isn't autistic, so that would have been an incorrect assumption.
However, to answer your question - rather than making an assumption, what I did do was ask him (neutrally) about his behaviour. He told me about his diagnosis.
If he had not known about his diagnosis, what most likely would have happened was that he would have apologised, promised not to do it again, failed at that, made me feel like he wasn't interested in my opinions and we'd have split up.
If he had been neurotypical (and just a rude fuckwit) he would have apologised insincerely, done it again and we'd have split up.
So you see that his diagnosis really did help me to understand him and love him. As did my not simply "assuming" his behaviour was down to being neuro diverse. (After all, if he had been neurotypical, I'd have ended up going out with a rude fuckwit and assuming autism. That's not good for anyone).