While Friends season 3, but that's not the point of the issue. The ABC sitcom followed the lives of six 20-somethings making their way in New York. The series focused on the group figuring out both their professional and personal lives, with every single one dealing with their own romantic experiences and hangups. That includes some of the gang dating and/or pining for each other.
Monica (Courteney Cox) and Chandler’s (Matthew Perry) love story came as a surprise to many. Their pairing was supposed to be a brief plot point and was only meant to be a fling. But, as the viewers resonated with the story, it eventually became clear they were meant for each other. Deciding to be a couple came easy for Monica and Chander and despite having their own issues, they tackled them together. That's very different from Ross and Rachel's complicated and dramatic narrative. While they were set up to be a couple early on, they were only legitimately together for just around a year over the course of Friends' ten seasons.
Ross and Rachel had a lot of memorable plotlines, but arguably the most talked-about was whether or not they were on a break when Geller slept with Chloe (Angela Featherstone), the girl from the copy place. During HBO Max's Friends reunion, everybody from the cast decided they were indeed on a break, siding with Ross' justification for what he did. While, technically, he didn't cheat on Rachel since they were on a cool-off period, that doesn't necessarily mean he didn't do anything wrong either. Whether they were on a break or not wasn't the main issue. It’s the idea that Ross took the first chance he got to sleep with someone else just because he and Rachel had some problems. What made this much worse was the fact Ross was the one who accused his girlfriend of flirting with co-worker, Mark (Steven Eckholdt), when all Rachel did was be a professional and work hard.
Looking back, it's unfair to simply zero in on the question to fully understand the situation. Before Ross and Rachel even got to that point in Friends, they were already having relationship issues. This could all be traced back to when Rachel started her job in the fashion industry after waitressing at Central Perk for years. Being that she finally got a chance to make a living out of something she was genuinely ionate about, it's logical she was working doubly hard to prove herself. This, however, frustrated Ross, as it meant less time for them to be together. It didn't help he was already suspecting that Mark was planning to make a move on her. While Rachel maybe could've better communicated her situation to Ross, it's difficult to fault her for wanting to excel at work.
So, it doesn't really matter if they were on a break when Ross slept with Chloe or not; what he did was still wrong. Rachel's decision to not get back with him was justified even if he technically didn't cheat, as it made her realize the absurd double standards he was enforcing on her. Of course, Ross and Rachel eventually got back together at the end of Friends. That doesn't mean their initial break-up was pointless, however. They both grew as individuals during the time they were apart, eventually making them realize they ultimately wanted to be together.