I Can see you did not play that update and it shows....
So lets clear up a few things
1) MC and Rachel never became an official couple.... lets just say it's complicated.
2) She never broke it of with Steve (Or told the MC that she will break it off with Steve. In fact, it's the opposite. She constantly mentioned that she still has a boyfriend. When the MC and Rachel finally have sex, it's in fact just a few days before they found Steve. (It was about the same time the made the tope that would be needed to climb the cliff)
3) Rachel did not just go back to Steve without feeling guilty about the MC as if it meant nothing. She was still constantly thinking about the MC.... even to the point where she was thinking of the MC while having sex with Steve. But for all intents and purposes, Rachel and Steve were still a couple.
And to be blunt: It would be weird for anyone not having sex after spending weeks apart.
I mean it kinda does. Imagine if all the military men that went on a tour of duty, came home and their GF just broke it of with them, or wives just divorced them.... I would call them shitty GFs.
And that's the crux of it. Rachel cheated on Steve with the MC. Not the other way around. (The fact the Steve also cheated on Rachel is irrelevant. Rachel never knew about that.)
I mean
R+S = couple ----Steve goes missing----shit happens ----Found Steve---R+S = Still a couple.
I mean how shitty would Rachel have to be to just dump her BF that has been "lost" for weeks for no reason at all. It's not like they had a fight or that he changed.... she just cheated on him. So if anything she felt guilty.
She did not know, so she still cheated....Come on, it's not hard. The were still a couple. The fact that he is a cheater does not change this. They both cheated.