In my opinion writers-Cowlip did not go with the wedding because they needed/wanted to have Brian alone at the end, for 2 main reasons: 1. for them Brian was the symbol of the gay community and of gay people so they wanted him to be dancing alone in final scene, because if he would be with Justin he would be perceived mainly as Justins partner, not as this symbol- proud unapologetic gay man, and it would cheapen the message they wanted to convey 2. They wanted to keep diversity of gay relationships. Michael and Ben as married couple on one end of spectrum, and then Justin and Brian on the other end. Brian and Justin for Cowlip represented couple who does not care about heterosexual and their rules, who does not need marriage (Justin cared in beginning of s5 but by the end its him who says we dont need rings...). BUT at the same time writers believed in the strength of Brian and Justins love, they said themselves they are bonded forever, thats why they gave us proposal, ILY, thats why Brian keeps the rings etc. So everything that happened in the last episodes has to be explained from these two goals Cowlip had. They did not care so much about logic of the story or if characters were OOC etc. If Justin would not go to NYC, they would get rid of him in different way, if you understand what i mean, simply to have Brian alone at the end no matter what. They accomodated story so it served their goal, their message. The political message was their priority BUT they could not deny strength of BJ relationship so they gave us ILY, proposal because they wanted to show us depth of love between BJ. IF there would be no proposal, we would not see how much Brian transformed and that his love to Justin was everything to him. Thats why the end is so messy and heartbreaking because writers needed to accomodate these two somehow contradictory messages. And although their "political" message took priority, they did not betray BJ love because Brian keeps the rings and says its only time in their last scene which means that eventhough wedding did not take place, he is still in love with Justin and commited to him, so they can have future together. I Believe these are the real reasons why there was no wedding and Justin left. They tried to tell us Brian was not ready and Justin has to make career etc., but thats just pretext, that was not real reason imo, on human level even writers could not deny Brian changed, is not anymore Brian of season 1, they even confirmed that his last dance in Babylon does not mean he reverts to being club boy. And he could be even satisfied with Justin in monogamous relationship. But they could not give us typical happyend simply because Brian as symbol of certain lifestyle, certain attitude of queer identity was for them more important than Brian as human being. Although this explanation of the end gives me some sense, it does not help me much to reconciale with the 513 and I am crying like a baby anytime I see it and my heart breaks down feeling Brians pain.