I feel like you are being kind of unfair with Ian, Cindy hasn't broken up with Wade yet either. Also, you don't have to do anything wrong to be cheated on. As far as we know, Ian did nothing wrong and Gillian cheated on him and left him for the other dude. Maybe the deciding factor is whether Lena fucks Axel or not, because in this game if Axel wants to fuck Cindy, he will succeed no matter what. Personally, I wouldn't like it but I'm sure plenty of Lena fans/Ian haters and cuck lovers (also known as patrons) would enjoy it very much.
Yes in theory Cindy is still with Wade. But there's no evidence that she's still sleeping with him. In fact everything suggests, their relationship is now so broken, they barely even talk to each other, never mind actually getting intimate together. If Ian is having sex with other people besides Cindy, then obviously that's a completely different situation to the one between Cindy and Wade, who are basically just treading water, before their inevitable break-up. Wade's depressed state during the beach scenes, is clear evidence of this. Things are not going at all well between him and Cindy, and he's convinced they're going to split. So in that instance, I don't think I'm being unfair with Ian. He knows the situation between Wade and Cindy is delicate, and for the time being he's accepted that he has to wait, before him and Cindy can openly become an item. So if he has deep feelings for Cindy, why would he want to sleep around with other women besides her? If he does that, then he only has himself to blame if Cindy ends up doing the same thing with Axel.
As far as Ian getting cheated on. Well, we don't know the whole story of why Gillian cheated on him. Whilst on the surface it appears that Ian did nothing wrong, Ian himself may not believe entirely the story he's told other people. Because why would he keep torturing himself with thoughts, where Gillian taunts him about his inability to satisfy her sexually? It could be just his own insecurity, caused by the shock of being betrayed by someone he never suspected could be capable of such a thing, or maybe there was an element of truth, and Ian really wasn't giving Gillian the kind of attention she needed? Because in this game, he can change and grow as a person and a lover quite a lot. So maybe he really was lacking as a partner back then, and this played some role in Gillian cheating on him? I'm not trying to claim Gillian didn't betray Ian badly, because she clearly did, but perhaps there was more to it, than Ian was willing to admit to his friends? ( Remember Cherry's actions with Axel, weren't as one-sided as Lena tries to portray them, so perhaps Ian's view of Gillian's actions are not as straightforward as he would have other people believe?).
I think where Ian and Lena cheat on each other, this is clear evidence of Ivy and Jeremy's assertion, that neither of them are ready for a serious relationship. They cheat, because one or both of them want to carry on having sex with other people, and their belief that they're in love, and want to make a commitment is not real, or as real as they believe it to be. Whereas, if they have a relationship and the players don't have them cheating, then that's proof that their love is real, and Ivy and Jeremy were wrong. In the end, the truth is what the players decide for the protagonists. And Lena is only a cheating whore, if the players make her into one. Ian can only get cheated on, in this game, if the player decides that's going to happen, by making choices for him or Lena. And, if you didn't want that to happen, why would you make choices, that would ultimately lead to that?