I'm glad they never kissed while Justin was with Ethan. That would have made me very, very disappointed in both of them.
Justin sneaking around behind Brian's back with Ethan was a big glaring sign that Justin was not being himeself. It was a betrayal of the promises they'd made to each other, and who they were as people. But Justin was in a horrible place then, and Brian had turned into a total dick and wasn't communicating, so I was able to not exactly forgive Justin for what he did, but understand it.
But if Brian had done to someone else what was done to him, or if Justin had done it again when he didn't have the PTSD and not-talking-Brian to jusitfy it? That would have really sucked.
So me, I'm SO glad they never kissed or did anything until Justin had left Ethan, and that Justin left Ethan for his own reasons, and not to go back to Brian. Much healthier and cleaner for all. :)