True, but like I said only when you started dating her when it was already over with Chad.
If you dated with her before that moment, it should end badly between them as well because of her cheating.
		
		
	 
This would make a great deal of sense, but there is one flaw. Taking care of Sage when she is sick. There are two versions of it. In one MC just takes care of her, but in the other he opens up about how he doesn't like his birthdays because of his mom. You only get the more meaningful one if you are fuck buddies with Sage. It doesn't fit with a "cheaters will never be happy" kind of morale. And the question of what actually makes one a cheater is present everywhere in the game. The message already is that there is a lot of nuance in the matter and you have to consider the details.
Sage having cheated on Chad is in a gray area. She had evidence of Chad cheating on her. Sage had already found the panties when MC met her and she didn't go beyond a drunken kiss before she had the video. And of course Chad was lying to her from the beginning so she really owes him nothing.
The problem is that Sage herself considers it cheating. How will she deal with it? It could be that Sage will carry the guilt forever and that sours her relationship with MC or she stops caring about cheating altogether which eventually blows up in their faces. If she however actually deals with the lingering emotions she has about it, it'll become an experience she learned from and their relationship could even be stronger for it.
The set up might even work the other way around and it'll be harder to get a good end if Sage doesn't cheat on Chad. She'll have the moral high ground and won't be as forgiving in matters where cheating Sage would be a hypocrite with the same attitude. The message would then be that we are all only human and setting too high standards for other people will just lead to loneliness even if you manage to hold to them yourself.