The potential for either partner to cheat on the other is always present, in all love relationships, at all times. No amount of promising not to or regretting having once done it ever removes that potential. It can't be made to go away by any means. All lovers can do about it is, on the one hand, chose to act with their partners'interest in mind and, one the other, trust that they will.
That's the whole point of being a love relationship. To grow beyond the narrow confines of self-interest. Whether that self-interest revolves around greed (cheating) or indignation (over having been cheated on and lied to) is irrelevant. Both extremes are expressions of the same self-rightgeousness and have nothing at all to do with loving anyone other than ourselves.