You don't know that.
People who are in unfulfilled relationships cheat. People in healthy loving relationships stay strong and withstand the test of time. This is not a new concept.
My take is Tully cheated because she was unfulfilled in her partnership with her girlfriend. Sure the right way to end an unhappy relationship is to address your issues, hold off on your lust, break up with the person, wait the appropriate length of mourning time, then whip off your knickers and throw a leg over the tempter and ride them like Sea Biscuit, but in the real world it usually takes the introduction of a third person to shatter and already delicate relationship, like we have seen with Tully, Tahlia and Drew.
I am not saying living through the experience doesn't hurt, I am not saying it is right for someone to be in a relationship and yet be so drawn to another person they can't think straight, I am merely saying it is a normal flawed human response.
Having lived through being the cheater and the cheatee, neither is an experience I would consider pleasant. However it is simply another upsetting life experience for you to reflect and learn from. No one likes intentionally hurting people they care for, but it happens. Life can be complex.
I too speak from experience.