I like that they become actual partners in her stories and that they both want the same things,with a bit of compromise on both parts.
It's so much better than reading stories where Justin is the submissive person in the relationship,just going along with whateever Brian wanted.
I don't think that the idea of them being monogamous in the future is necessarily OOC. There are a lot of people like Brian who do eventually decide to be with one person as they get older. Few people are exactly as they were when they were in their 20's and 30's,in spite of Lindsay's calling Brian Peter Pan I think that would include Brian,jmo.