Okay, I found a few more boxing titles. The reviews are a bit mixed on some of these but I figured I would at least mention them.
World Boxing Manager (PC/Mac)
World Championship Boxing Manager (PC) - a classic game port
Punch Club (PC/Mac) - more of an RPG/Strategy game
Title Bout Championship Boxing (PC/Mac/Linux/Android) - outdated but still has a small community of people playing it
World of Mixed Martial Arts 5 (PC) - I know it's MMA not boxing, but I still have to mention it because of how detailed, in-depth and realistic it is. I've played hundreds of hours on it, mostly thanks to such a fantastic community with superb modding.
If I track any more titles down worth mentioning, I'll let you know.