I think if Justin would have messed up in a lot of different ways it would have been easier to say that most of the blame shouldn't lie with him, but Justin kept making the same mistake over and over as the relationship between Brian and Justin went on. I think that is why Brian became indifferent to the relationship and cut off communications. I think that at 30 it would become exhausting to constantly see someone you love cross the boundaries that they made you set. While wearing the hat of caregiver and mentor at the same time. Think about how conflicted Brian must have been knowing that yes Justin was breaking their rules left and right, but also not wanting Justin to be held back or allowed to explore his life a growing man. It wasn't just Eric, it was The Sap and then Ethan.
All window dressing things. The Vermont Trip, The hustler, The picnic sure they might have been disappointments but they were material things that could have been fixed or objected too or even swept under the carpet. They were breaking the fundamental building blocks of the relationship like Justin breaking those rules were. To me Brian did have fault and flaws but they were of a different brand they were of the "I am new at this relationship stuff, I can get better" Unlike Justin's that were "I cross boundaries, and don't really care"
The only think that Brian did, that was really wrong, was peeing on the art work that Justin and Michael did for the comic book. It was wrong and crossed a lot of lines. I think about how long it must have taken Justin to do the work and the effort that Michael put into the stories and I can see why that was an issue but the other stuff that Brian did can be forgiven.