Completely depends on the style of game but I noticed I drift towards game that implies the girls for the most part know of the other girls or MC being a complete horndog.
If forced in the matter I would say I stay loyal but usually do another run where it is full on cheating.
IME games don't reward you much for staying 'moral' or loyal. In fact, most do the opposite. Games tend to follow the "thinking" of the creator and not an average citizen's. The creators 'perception' of what 'normal' may be. So, it's generally a walk down a writers fantasy of what is logical, moral or accepted behaviors. In short, sometimes you just have to do what they want you to do. Then there are some games that are so well done that you must play the straight man to the screwed up world and hate yourself for not not being more moral.
Depends on the tone of the game really. Sometimes they're just fuck fests and other times I'd feel bad for cheating. Like the Bella/Jill route in BaD makes you feel like complete shit if you take too long to make a decision.