MC not being in a serious relationship is not cheating.
However since he's dating Jill and fucking behind her back it kind of is.
If they had an open agreement that he can fuck around then it's not.
But that's not the case, so he IS cheating her (not ON her).
The sauna scene with Bella is a good example that it is.
They stop so they would not get caught, if it was not cheating they would not have care, regardless of Jill's take upon the knowledge that they are playing around.
Beside just because some doesn't think Mc is not cheating means that the girls will think the same way.
He's not cheating on Sage that's for sure, but I don't think Sage will be happy about it, because she has feelings for him and from her PoV Mc has some too for her.
Holding hands, hugging real close meaningfully, the kiss at the end of episode 5, all that show that's it's more than a simple FwB relationship but none of them acknowledge it ( MC does wonder though).
Now with Chad's cheating and her new feelings for MC she may consider it cheating and neuter him.
Since I'm talking about Sage, why are people already speaking of her leaving college and/or dumping him?
The year just started and a lot can happen in a year, heck some people marry just a few months after meeting for the first time.
They can still be together while Sage is working and MC is studying.
Age gap is just a bullsh*t pretext for them breaking up if they were together.