Yasaotoko to Sadistic
A man becomes obsessed with a former classmate he used to bully and stalks him for two years. Not a relationship you should emulate, but it makes for great fiction.
Ringo ni Hachimitsu
Two stories: As a child, Natsuki saw a gay couple on the beach, and it was his sole notice of them that made him realize he was different. Years later, he returns to the beach with his boyfriend. (There are more chapters about their love, how they met, and a beautiful rainy day at home.) Their story continues in the book Kare No Barairo No Jinsei. The second story is about a man who, fearing the end of the world, reconnects with a classmate he has admired for ten years. Their story is ambiguous, but I believe they will end up together. Some speculated they were the couple Natsuki had seen on the beach, but since their story takes place in 2012, I don't think it's possible given the timeframe and Natsuki's age.
Trap (OOTSUKI Miu)
Five years after a sensei blackmails a student into sleeping with him in exchange for silence about cigarettes, they meet at a reunion, and it turns out the sensei hadn't been in control as he'd believed.
Goodbye Lilac
A sensitive, well-paced, beautifully drawn story of Katou, a boy inspired by his teacher, who grows up to become a teacher as well. Five years after graduating, they reunite as coworkers, and Katou is still in love.
Seinen wa Ai o Kou
Two high-school students realize their feelings are mutual. A man reconnects with his high school's idol on the set of a gay porn video.
Eien no Shichigatsu
Shortly after getting engaged, Tachibana is reunited with a senpai from university, Ryuusuke, who asks to play "couple" for a month.
Oko-sama Box
After Fujita is spectacularly dumped by a girl who calls him impotent, Kawasaki, who has loved him for a while, initiates a sexual relationship to cheer him up. It lasts a few months but ends abruptly when Fujita declares he'll find a girlfriend in college, leaving Kawasaki heartbroken. Years later, they've had no communication, and Kawasaki is shocked returning home from work one evening to find Fujita outside his door. Tie-in story to Oko-sama Star, the love story of their two friends.
Hatsukoi
The second couple, two students who rent a room so they can be together, was the only story that really worked for me due to the lightening pace, but the collection is sweet, and I'm happy the apartments will soon have tubs! ETA in 2020: Past!me continues to have strange opinions. On a slow reread, this was cute as heck. The apartment manager and a childhood friend repair a friendship (and walls). Two college grads who live together in secret have to decide if they'll tell their families. A too-kind man who cosigned a bad loan falls for his charming debt collector. And two guys who had hooked up as roommates years before find themselves roommates once again due to some incredible coincidences.
Ii Mon Waru Mon