My boyfriend is friends with his ex?

A few nights ago, my boyfriend's phone buzzed and he said it was from his ex. It read, "hey, it's been a while. Was just thinking about you." After I found out what it said, he admitted she had been an on and off relationship a while back and they remained friends. It was midnight, and I made him reply saying he had a girlfriend. Was this wrong of me? He says her intentions were completely innocent, but it's hard to believe that an "on and off" ex would text him at midnight with innocent intentions, especially when there was no previous ongoing conversation. I do have guy friends as well, but when they texted me at that hour (we had been talking throughout the day) he told them to back off of me. Somehow I just can't believe that she's only interested in a platonic relationship; I can't even see why he'd want to maintain one with her since she treated him so poorly while they were dating. He told me to not text back and to leave it be, but I'm not sure that I trust him to text her back when I'm not there, and that worries me. I felt that I needed to be there to make sure she knew he was seeing someone. Is this normal? Am I controlling?