Too many opinions for me to quote here, but my take on Gillian's motivation is youth. Living life and having relationships builds a base of experience, most if not all of the characters in this are under 25. Like a lot of people do, she thought the grass was greener outside of her relationship with Ian, and might have found the opposite.
I think Ian's dreams are nightmares, it's possible she never slept with anyone while they were together, Ian just making the assumption it was the worst possible scenario. Given Ian's friends reactions, she may have cheated, but they are Ian's friends, and prone to believe him and take his side.
Mike, as far as we know, slept with 2 of the characters in the game, Lena (player choice) and Ivy (to the dismay of her fans), 3 if you count his girlfriend. Hardly a serial cheater, but a cheater nonetheless. He might have been Jeremy before his current girlfirend, but since... or maybe he can't change his spots.