i have always said that Brian would never have accepted Justin and the concept of forever with him if there hadn't been Ethan. so i'm not arguing that it had to happen. It did, and it was important, but mostly to the growth and development of Brian and Justin's trust in themselves and each other and their relationship together.
So ethan was not something to be ignored but something that, in the long run was of so little consequence that Justin doesn't even factor that as time apart when thinking about how long he and Brian have been together.