I guess he was too much in a shock to have run away immediately. I doubt he predicted Hiromi would right away come his way after saying those words.

It was interesting development to say the least. Shinichiro was Hiromi's ally in the family, with his mother disliking the fact Hiromi is there and the father probably ending up being somewhat neutral in order to not piss off either side. I wonder how it will go on from now on. Although Shinichiro doesn't look like a scumbag who would now make things hard for her, but who knows what's going to happen. She was his dream girl after all.