to me, it is completely believable that robert would lie to himself about his relationship to kev, and that he would lie to aaron. all he has wanted since he got back was to get back to the way things were six years ago, he wants to forget the past six years and just get back to where they were before. admitting to aaron that he's married, admitting that kev is violent (to others) and that it was a marriage for protection, for comfort, reveals something about how much he has changed and robert doesn't want the past six years to have changed him. he's scrambling to get his life back and, to me, it makes sense that he would want to keep those parts of his life separate because introducing one to the other not only reveals the new relationship, but it brings those six years out into the open for all to scrutinise and puts the relationship in a new light. I think robert knows this relationship with kev isn't healthy and good, but he's trying to brush over it because admitting anything else is admitting how hard it was, how bad it got.