Why would people hate Sage for that? Chad is a little pussy boy who couldn't take care of his girl because he'd rather suck dick. Fuck him.
Probably in their mind a girl shouldn't cheat on a guy while she's in a relationship. That's funny to hear considering Josy and Maya cheated on each other as well as MC himself.
Perhaps their life experiences also influence their perception of the character.
I would very much disagree with the sentiment that both didn't get any development after ep. 7. I think the love triangle is still the most interesting and dynamic relationship in the game, especially because it already shows some inherent cracks and problems ahead.
The other main paths are basically finished now. The Bella path will likely see some trials and tribulations that mostly come form the outside due to it being a teacher-student relationship and maybe due to the age difference, making it the 2nd most interesting relationship. But the Jill and Sage paths are basically done as far as we can see. The only foreseeable interesting developments can come from the outside, like Jill's parents objecting the relationship. And with Sage I can't see any problem whatsoever (at least as long as the MC didn't had an affair with her mom).
So having the tuition problem move into the background and focusing on the problems any love triangle can produce could very much lead to the most interesting story bits of the rest of the game (if done well obviously) and make the J&M path - at least in my opinion - the most intriguing one of the main paths.
Since the beginning of ep6, they've been acting like a troupe and the relationship is pretty much the same. In episode 7, Patrick's conflict arose, which led to an unofficial rivalry situation, but otherwise everything calmed down.
My main complaint with this route is how sluggish they are in reacting to the fact that MC may have a relationship on the side. We only get a couple comments and that's where their response ends. They are just waiting to see what decision he makes and this is implemented poorly in my opinion. Jill dumps MC when he ignores her at the picnic, Bella when she finds out he had fun with Cathy, Sage basically never saw him as a boyfriend,as a boyfriend, just as a dude she just fucked.