I don't think it's about getting caught in those specific moments, or any of the girls spilling the beans to your girlfriend, but just building up to the point that MC will actually cheat. Say, needing 10 cheat points to fuck Cammy while dating Jill. You already fool around enough that here you can go all the way, whereas 5 points won't be enough to ignore your conscience and you'll feel too guilty to cheat.
Besides, when you're on a Main Girl path you currently cannot do anything with others. Can't fuck Cammy on the dance floor, or accept Heather's bj, or visit your favorite whore Madame Rose. The best you can do is be handsy and flirty.