I've been wondering and I'm feeling like it's supposed to be first love as in a first but unrequited love that is bound to not succeed. Kinda like that first crush you have but nothing's going well. Both for the longest time tried to forget/deny this feeling but couldn't.
Could be wrong, but that's how I understood the title: the end of Katou's love being unrequited. It's not a lonely feeling anymore because they're together now.
That's interesting and makes sense. I feel the title could be too because sensei had to let go of the student to be with the man. I mean, his coworker mentioned something about affection turning into love, maybe they are saying goodbye to that initial feelings and welcoming something more beautiful.
Anyway I'm glad it's a happy ending, that title was misleading.
I've been wondering for a while, but why goodbye first love? *SPOILER* Both of them got together with their first love. I feel I'm missing something important.