I have a question is the cheating tag is used for
A girl cheating on mc
Or mc cheating on his Girl??
It's MC cheating on Jamie, but it's optional, and it's only at the beginning of the game, up to the point when Jamie and MC has actual sex for the first time (middle of day three). After that point, things will be somewhat different. If you've played the game until you're past this point, you'll know what I mean. If you follow the "Cheat Path" in OfficiallyGames' walkthrough document, you'll start off cheating on Jamie. You'll have more lewd scenes in that path, but you'd be cheating on your girlfriend, at least at first.
Personally, the "Loyal Path" makes me feel better about myself, but more recently, I tried the "Cheat Path" as well (just to try it out), and it will give you all possible lewd scenes, but I felt like a jerk at the beginning.
Also, relationship-wise, only Jamie and Shauna will be "mandatory" (for Shauna, you get to choose when this starts, more or less), everyone else is optional (or, rather, can be avoided), if I remember correctly. But, given
the deal that Jamie sets out for you right after you have sex for the first time, you might as well go for it.