It's not the place to discuss it here and possibly foul up this thread, but if someone who is Japanese could kindly PM me and explain just why that is offensive, and what the preferred shorthand is, I'd appreciate it, as that's always blown my mind as well.
Or if a non-Japanese person knows the clear cut answer, that'd work too.
Anyway, if you didn't know about it, don't worry about it if you don't do it again