They weren’t even on a break.

Rachel confirms it the next day tho in TOO the morning after when she says, "they were on a break" as if it were temporary.

I agree that regardless of the break happening or not (it did imo), it was bad. But I think the whole point was that the writers had to find a way to break them up without making Ross look that bad, hence the whole, "we were on a break" thing. I also think Mark being at Rachel´s place (whether it was her fault or not he was there), when Ross tried to talk things over with her makes what he did more forgivable, of course that is barely ever brought up.

"The point was Ross slept with another woman....just hours after their little split. It does not matter if they all understood it to be a break. There is no way anyone would get back together when one of the parties had sex with someone during a 12 hour tiff break up."

I think pretty much everyone can agree on that, I think Ross even understood that. Although I have to point out that it wasn't just a tiff break from the perspective of Ross. Well, it was, until he discovered Mark was at Rachel's apartment and he thought she had dumped him for good.

The continuing debate about whether they were on a break or not actually goes back to the episode where Rachel agrees to take Ross back on the condition that he admits he cheated on her. This is simply not true and there's no reason for Ross to admit to this, especially since it absolves Rachel of any wrongdoing in their relationship.

It did come across as a little hypocritical from Ross, after he'd spent what seemed like forever blaming Rachel for her friendship with Mark. Whatever Mark's intentions were, Rachel hadn't done anything with him that would cause Ross to be so jealous. Yet within a few hours of having a row, Ross had jumped into bed with another woman.

As an audience, we saw how upset Ross by the argument and that he'd done something out of emotion and stupidity. But from Rachel's point of view, you could understand why she'd see it as a betrayal. It must've felt like Ross had jumped into bed the second he thought he was free. That probably wasn't Ross' intention, but does being drunk and upset justify jumping into bed with somebody else after you've spent a long time berating your girlfriend for a platonic work friendship? It's not like Ross even waited to see if the argument was permanent before getting with somebody else.

Bear in mind, in a later season Ross met Mark and commented that he thought the guy; "seemed nice" before he knew who he was.

There's two sides to everything, but it's not as simple as saying they were on a break, so Rachel was justified.