This is news to me. My apple ID is on my personal domain (Google Apps) with only one email address on it, and it works fine. Could it be a recent change?
About 6 months ago I was unable to use my email on a personal domain to create an Apple ID. I had to create an @icloud.com address. And I don't think it's an issue with my domain, it's been fine for the last 9 years.