Ross and Rachel were absolutely on a break at the time, and Ross was fairly certain Rachel was spending the night with someone else. He was absolutely in the clear for sleeping with that girl.
Which makes it frustrating that they focused on it being a break, because everything else Ross did was completely in the wrong.
He was overbearing and paranoid, regularly invading Rachel’s life to assert his dominance in front of guys who do not care. Rachel was regularly humiliated by him, and you can’t blame her from needing time to recover.
After the incident, Ross made bold lies straight to Rachel’s face, and spent the entire episode trying to keep it a secret. Finding out about the incident from Gunther was the icing on the cake of humiliation.
Then later, Ross told yet another lie to Rachel in pursuit of what he saw as easy sex with his dream girl, then claimed that he was justified for it because she was too wordy. And he only admitted his lie when he felt his ego was threatened.
Ross ruined the relationship because sleeping with the copy girl was the only thing he wasn’t in the wrong for.