Carefully read this sentence again. You might see the flaw. Gillian cheating on Ian is the root of most of his struggles with self-esteem. She hurt him the most by a mile.
At no point in time is Ian in relationship with either Cherry or Cindy. So they can't really cheat on you. Cindy is a terrible person in general (and Ian has to be a backstabbing friend just to get to her as well, only for her to then turn around and fool around with roid rage blondie and never really commit).
Minerva is a married woman and a lot needs to happen in the coming chapters in order for their romance to work out. She made Ian's life hell in the beginning for no real reason other than being unsatisfied with her own life. Moreover, she's not everyone's cup of tea characterwise.