Well I also don't like cheating themes in mangas, however on a cheating scale the most impersonal form is orgy because it is solely based on sex, it is even a bohemian, erratic and circumstantial (under the spur of the moment) form of sex. The most personal form of cheating would be having a mistress or a lover along with the "official" one.
At first I didn't like this particular piece of news, I didn't see it coming given the fact how much Katou is possessive of Iwaki-san. However, after listening to Katou's explanation and taking into consideration that he has a childish part in him, it is no surprise that he cheated and hid it. From his perspective, it wasn't "cheating" because back then he said that Iwaki was not reciprocating his feelings, the thing which was making him unhappy and drove him maybe into despair. It is true that he moved to Iwaki's apartment but for him he wanted the same intensity he was showing to Iwaki (he said that he was being "cold" towards him back then).
However, being the person he is (with that childish aspect in his personality) he retreated into a familiar shell (sex) instead of working his way through his "problem" and waiting till Iwaki opens up to him the way he wanted to.
From his perspective, he had doubts (maybe Iwaki didn't love him, maybe this "thing" wasn't going to work out), hence the orgy was supposed to be a test (maybe after doing it, he will discover that all his feelings for Iwaki were an illusion - after all, and in spite of the fact that he used to be a "lady - killer", he has fallen for a man, which is unusual given his past - and if that were to happen, then he would have found a way out of his unhappiness).
This way of thinking is to be expected because all what Katou had in the past were flings and I don't believe that he was in a serious relationship before or that he had these intense feelings towards someone before.
If he were a little bit more experienced, a little bit more mature, he wouldn't have doubted his feelings and hence wouldn't have cheated.
For me this orgy "affair" is exactly like when a child is sulking and starts to break things out of spite.
Not telling Iwaki about it and insisting that he would never tell him something like that (even if there were other similar "mistakes") because he didn't want to see him hurt and crying, is part of this irrational bratful but still wonderful madness which characterizes Katou. Iwaki understands that and this is why he forgave him, especially after he saw his reactions when Iwaki was going to fall from the bridge.
For me Iwaki compensates for whatever Katou lacks, if he weren't mature, strong and generous (because forgiving is a form of generosity) this relationship would have never worked and if Katou weren't bratful (but still genuinely affectionate, caring and passionate towards Iwaki), Iwaki wouldn't have loved him back.
In a sense, they complete each other.
Hence this is a well-thought of and touching romance story (with adult themes to spike it).
Wow an elite school, that is very impressive. I don't think that parents's attention is detrimental to success (quite in the contrary it can become in some cases a liability). Of course parents have an important role in our upbringing but if their parenting is not working well, we shouldn't let this stop us from succeeding and being happy. I think that a person can become successful and happy even if he wasn't surrounded by his family's attention if he works his mind through it.
If parents made a mistake by not being equally attentive towards all their children, then it is bad but we can still love them as much as we can. I don't have any younger siblings but if I had any, I would be very attentive towards them if I thought that my parents are giving too much attention to the older one.
Furthermore, I have a little theory of mine - which isn't infallible of course - I think that children who learnt to mend for themselves make wonderful partners (lovers) when they turn into adulthood. I believe that they are more perseverant and wouldn't jump off the boat of a badly going relationship at the sight of first troubles. They tend to be more faithful, honest and appreciative of their partner because they learned to get what they want in life the hard way i.e. it wasn't handed to them on a silver platter. Of course it is just my opinion, it may not be true about everyone but I believe that it holds some truth to it.
"The Sunshine Family"? Wow, that's one hideous collection (I had to google it and it turned out that it was produced back in the 1970's with low-cost in mind in order to compete with doll makers who re-located their facilities to Asian countries).
Anyway, it is good that you've learned to depend on yourself from this. Even only children can opt sometimes not to depend on their parents. For instance, Danny Choo, Jimmy Choo's son, never accepted one penny from his wealthy and renown father (or so he says).
His dream was always to visit Japan, work with the Japanese and learn the language and he just did all that. He started his way up by waiting on tables in a restaurant which served mainly Japanese customers to finance his traveling projects. This is how he picked up the language too and he even ended marrying a Japanese woman.
I found his resume to be inspiring so perhaps your parents unintentionally gave you something much more important than the Barbie collection and cars... the ability to spread your own wings without help (perhaps you will be more apt at succeeding in life than other children who have been pampered by their parents and received their cars without hard work!).
Well I see ... eventually plots like paintings can be too interpreted from different perspectives. Of course, I see that you believe that Iwaki is too good for Katou. I don't agree on that but I respect your choice ^^
About the other matter, it is true that if the mangaka wanted to complicate things then Iwaki would have brought up the subject of that woman along with the orgy and expressed his frustration at Katou wondering how he can trust him or if he had said the truth about that woman (if he hid the info about the orgy then maybe he lied elsewhere).
However, Iwaki who was really absorbed in his thoughts about this matter, chose to forget about it saying that it is one of those "petty" matters from the past after he looked in the eyes of the startled Katou and his facial expressions at the moment he rushed to save Iwaka, risking his own life in the process.
From my own perspective, I think he forgave him and then made love to him because he thought that such affection couldn't be faked. It was genuine and that what mattered to him.
I won't drag this any further because I do respect your point of view but it seems again from Iwaki's words and actions that Katou is worthy of respect both on a personal and professional levels. He did say after all that he as an actor, sees Katou as a rival, and this entices him to excel and do even more. From the manga's progress we could see too that he has made an effort to become more mature. That was Iwaki's opinion of him ...
Could Iwaki have been better off with someone other than Katou? I say "no" but if you say "yes", then it is also a valid opinion.
I won't dislike your topic for this because I already voted with a like. It is the diversity of opinions between fans which makes the feedback interesting ^^