Maybe I'm wrong but to me it's pretty clear that Ian would have forgiven Gillian when it happened. The difference with Lena and Axel is that he didn't have a choice. Gillian didn't just cheat, he left him for the other guy and is still with him as far as we know.I mean, even if the relationship went LDR, she still cheated, though. Being together for 4 years, you know the sex becomes kinda monotonous. She spiced it up with cheating. Why would Ian give a cheater a chance?
Why Ian or any other person would give a cheater a chance?. It's something that I struggled myself to truly understand, but I probably have trust issues. I have not been able to even get into a relationship with someone who cheated on another guy with me. I can't stop the doubt of "if she did that to the other guy, why wouldn't she do the same to me?" and for me a relationship without complete trust makes no sense. Intelectually I can understand that some people can truly forgive other people mistakes and people can change and not cheat again. Also, some people are really terrified of being alone or too dependant on their partner, so they let them get away with anything.