Robert was always going to cheat; he is a known cheater, and in a soap, every relationship has a cheating SL in one form or another. By doing it this way, Emmerdale have definitively shown us two things.
Robert’s happiness and well-being is entirely dependent upon Aaron’s, and Aaron’s presence in his life. The days leading up to The Incident™ clearly showed Robert struggling; he couldn’t sleep, looked haggard and exhausted, and the only thing that brought him any consolation was the phone conversation with Aaron in the evenings. Finding out Aaron was in trouble for being gay made him look ill, made him worried sick. Finding out Aaron was on drugs made him almost desperate. His husband was hurting and self-harming and he couldn’t do anything about it. When finally they did fight, and Aaron left him at that table, he was devastated. Even if they didn’t show us that the fight was close to a break-up, the episode where Robert almost told Aaron about The Incident™ had Aaron admit that he knew Robert thought they were over, and as soon as Robert got it into his head that they were over, that he didn’t have Aaron in his life anymore, he went into self-destruct.
Robert has always been a self-serving person, and he has never hesitated to hurt someone if it meant he got something he wanted, but with Aaron he feels remorseful, he feels guilty. This has never happened to him before; not really. Robert is a person who lashes out when cornered, when hurt, and this was a classic case of Robert reverting back to the person he was before, hurting the person who hurt him by breaking up with him. However, in all instances before, Robert has shrugged off their pain, even reveled in it, enjoyed giving back as good as he got because whether or not he hurt people didn’t matter to him. However, he is not in a position where Aaron’s happiness is inexorably tied to his own, and this means that he cares about what he did, he hurt someone he never wanted to hurt, he has caused pain that is akin to hurting himself and it is quite clear on his face. He feels guilty, looks tired, looks like he hates himself (in my opinion) and both of these things are both down to the broken nature of their personalities.
Robert is dependent on Aaron for a home, safety, peace within himself, and the fact that the disruption of that peace comes across quite clearly on screen has, for me, cemented the fact that Robert has changed since meeting Aaron. He no longer hurts people haphazardly, unfeelingly, and without personal repercussions. Robert feels the pain acutely, can see that he has done wrong, which is a huge character improvement from Robert of a few years ago. This also tells me that Emmerdale probably won’t have him cheat again, at least not in the foreseeable future. It causes him too much pain.