You are arguing with quite literally the game's author, because it's EK who flags this as cheating in the game's script. When Lena and Ian have a fight that's just that, a fight between people who are in relationship. Lena acknowledged herself in her internal monologues that by fucking Mike she is cheating on Ian, but she is so pissed off at Ian that she does not care.If she has a fight with Ian that means they are not dating anymore. That doesn't count. Yes, when they have an argument outside of Blazer's about her Stalkfap, Ian leaves (they break up and never get back together) and she goes back inside and fucks Mike in the bathroom. That doesn't count as cheating.
Ian and Lena most certainly can get back together after that fight, it's nothing like "they never get back together".