Hmmm, so to add to this, do we think that Justin is sort of merging with Brian's perspective on not needing marriage, in the end? Cause of Justin's comment, "We don't need rings and vows..." He wanted marriage so bad, enough to leave Brian, but then he gets engaged and suddenly "We don't need rings and vows..."
Maybe the failed engagement brought them back to the same page? Seems to be a bit of a theme with Justin, "Oh, Brian's not giving me what I want so I'll just leave..." Then he figures out Brian already was giving him what he wanted, or Brian makes a grand gesture, and suddenly Justin changes his mind.
I also think the proposal was more to make Justin happy. But it was also for Brian, because he wouldn't have seen himself ever feeling that way towards anyone else.