NOPE! Neither of them have relations with anyone else.
Spoilers (LIKE A LOT. BE WARNED)
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I browsed the raws. I didn't understand a lick of jap. But I think the images were enough to conclude that:
- The sensei quits and leaves before it gets any deeper
- sensei doesn't have any questionable relationship with ANYONE
- the twin of the school nurse was someone special to him, who unfortunately died in an accident when they were in HS (i don't think they were lovers tho, but it's safe to assume they were special to each other)
- Haga became good teacher -> MATURED REALLY WELL (like seriously shed his playboy persona. Still friendly, positive, and fun loving tho)
- they met again by chance when Haga was already a full fledged teacher (Yura was no longer a teacher & was working in a restaurant)
- Haga tried his best to court Yura again (less impulsive and touchy. When he did lost a bit of control, he IMMEDIATELY said sorry unlike before in HS - nothing happend btw)
- they end up together! (Haga really helped Yura heal)
As for the school nurse: I'm not sure. His expressions were more complex and subtle so it's hard to guess what was going on his mind. BUT. He did have Yura's best interest at heart (and actions. Really. He didn't cross a line)
The overall tone of the manga is actually pretty bittersweet because of Yura's loss and Haga's heartfelt struggle to connect with Yura (It was SUPER sincere that it was heartwarming)
Ugh no pls don't be that type of comfort I hate that type of ba when we're shown uke or seme having sec with others