Saying that Akira doesn't cheat on anyone except Chika is just mental gymnastics and you know it. Chika, Ayane, Makoto, Futaba, Niki... They were all (some still are) cheated on behind their backs until they chose to forgive him, but they still don't like him plapping girls behind their backs
There's no way a real person that had relationships like that in real life and got caught would get away with it without severe consequences
It's not mental gymnastics. You can't cheat on someone with whom you are not in a relationship with. What Akira does isn't cheating, but grooming/manipulation. If both sides are adults, then either can have the maturity to say "well, I wish for a monogamous relationship, and if you don't, that's a dealbreaker for me". However, Akira, by engaging with teenagers, is purposefully taking advantage of their lack of maturity by putting them in a position where it's clear they want to be exclusive, but don't have the maturity to stop seeing him either. That's one of the million issues of an adult engaging with teenagers, the other part simply doesn't have the maturity to be in that relationship in the first place.
Sara (an adult) calls Akira her boyfriend, is he cheating on her? That's preposterous. He isn't her boyfriend - and has told her, and people close to her that, multiple times. However, with adults it's a clear-cut situation because they choose to be in that situation, while having the minimum maturity to get out of it, so it's on them. What you call "forgiveness" is misguided, forgiveness could only be applied in the situation of Chika discovering Akira's cheating on her, and forgiving him for it. The other girls can't forgive Akira for anything, because he hasn't agreed to anything in the first place, so what's there to forgive?
I suppose "I forgive you for not correcting me in thinking I was the only one" is possible in
some cases, but even then that wouldn't be cheating, just manipulation again. Important to note that Akira's a piece of shit regardless, just for different reasons than cheating (most of the time).
As for real life, that's basically what happens everywhere all the time. People don't usually start seeing eachother and automatically are boyfriend and girlfriend, during that stage it isn't uncommon for them to be getting to know multiple people at the same time until settling on one, and that is doubly true if they're younger. In fact, a reasonable amount of times the need to "define the relationship" comes from the relationship having advanced enough that someone feels like they wouldn't be okay with the other person seeing someone else, and calls for a meeting so that becomes a pact between them.
