[MMM]

I agree with what Nyororin said.

I think when you had a boyfriend you were unattainable, and therefore more accessible in front of the students...i.e. he could be mockingly flirtatious in front of his students to satisfy their desire to have their single teacher find a lady. (This is pretty common in my experience. And I am sure he didn't put his students up to asking you if you have a boyfriend. He didn't need to. That isn't considered a private question in the mind of Japanese students.)

But once you became single that kind of classroom play may have felt inappropriate. I can't say what his feelings are, but keep in mind he might have been acting initially on the encouragement of his students.