I finally heard back from Mr Ex. It was a pleasant e-mail. He apologized for asking that dreadful question:

i appologize if the question i sent
in the last e-mail was innappropriate, but it was a
concern for me and something that felt needed to be
asked. don't ever feel like you have to answer it if
you don't want to. it was more one of those nights and
all kinds of things were swiriling in my head. as far
as our friendship goes, i'd like to believe that we
will always have that.

Hm. He doesn't seem so confident about that last part. I'm not either. At least I have a way out of causing any long-term damage that would come from answering that question.