i see a lot of people talking MCs actions in the end seem unreasonable, and since i wrote a god damn paragraph on my opinions, feel free to let me know anything you disagree with, sorry for the length and bad grammar
In the beggining of the arc that leads to MC cheating, we see him fall very far into depression and begin a spiral downward. In this time he is rejected from company after company blah blah blah he is very depressed. In this time he stops going to clubs stops confiding in anyone. He is trying to make his dream come true and his whole life has been leading up to this one moment where he fulfills his dream of being an editor or whatever, and his whole life had gone fairly smoothly, he isnt used to failing in something he actualy wanted so badly. This begins MC keeping his feelings bottled up, now the key thing here is that he doesnt have anyone else to talk to exept bitchress (girl he "cheats" with), the reason for this is that among his friends and gf he is awesome and amazing and always does his best ect., so he most of all doesnt want to disapoint them. Therefore he talks to a girl in the same boat he has never met before, he doenst love her at all and doesnt see her romantically in the slightest. You may think well why does he go on the trip than, the reason is simply to run away. He is just doing anything to avoid his gf. Also notice he see bitchress as his gf because thats who she really wants to be comforted by. He was using sex as a release for all his stess since he couldnt talk about his feelings he used his dick too. Also noter her menthioning his dead face, this is more evidence of him bottling up his feeligns of dpression and lack of self worth. He couldnt face his pregnant gf when he was too worthless in his own opinion, so he ran to talk to someone about all his problems again. Thusly, i think it was complelty in line with MC, as his rich sheltered shy self and his emotions matched well with his selfish selfloathing actions. sorry for this being so long, i really really liked the amazing writing here and had to rant about it.