I don't remember how the math works out exactly, but I think lust of both participants (GF and the futa) affect the odds of sex events firing, and the affinity differential (with futa vs with mc) plus cum addiction affects the odds that the GF will go along with it and cheat.
iirc once affinity to a futa is higher or equal to affinity with the MC, then the GF will be willing to cheat. This means that once futa affinity reaches 100 you cannot prevent the GF from cheating no matter what.
Maybe there will eventually be an ending where you can beat the game fast enough before the GF cheats, but right now there is no real point to trying to prevent it even with save-scumming.