Kurumi: I won't have sex with you. Even if I do it with someone else... or as a casual fling... even if we were the last two people on Earth... I'd still never have sex with you. No matter how much you cried and begged for me... I'd never let you come. Are you... okay with that?
Aiba: ... yes.
Sundome (Stopping the Moment Before) is a Seinen manga by Kazuto Okada, who also authored Kyoukasho Ni Nai.The main character is Hideo Aiba, who believes that allhumans have specific turn-ons and, because of his specific demands, he has never "fallen in love with someone at first sight". Enter New Transfer Student Kurumi Sahana. She is odd, yet "all of her" is Aiba's turn-on. She gets a seat next to him, and the other students note Aiba's lack of interest in 3D girls. After school, Aiba is a member of the "Roman Club", whose members search for UFOs, paranormal activities and the like. The Roman Club was funded by school alumni now involved in all facets of Japan's politics and economics. Also, every member has a specific fetish and losing your virginity means you get kicked out of the club. For those members who uphold the rule of remaining a virgin until graduation, the alumni supply numerous recommendations and backups for getting into colleges and high-paying careers. However, due to so-called assassins, women sent by Roman Club alumni to cater to the Club members' fetishes and rob them of their virginities, six years have passed without a single member successfully graduating. As Aiba tries to get out of his membership, Kurumi shows up, showing an interest in the club. After the other members leave, Kurumi starts reading "The Secret Book of Masturbation", which she found in the club, and asks Aiba, "Show me how you jerk off." This is the start of an unusual relationship.
Contemptible Cover: The covers of the volumes and the individual collars often have Kurumi in poses and dress implying themes of BDSM, sexual abuse, and violence that never show up in the actual manga.
Creepy Doll: Tatsuya's doll that he carries around constantly; he says her hair actually grows.
Cute Little Fangs: Kyouko has two which are almost constantly seen when her mouth is open, giving her a very vampire-like look.
Dark-Skinned Blonde/The Ditz/Dumb Blonde: Kyouko is blonde, has a perpetual tan, and is generally dumb as a bag of rocks, although she can be quite canny about some things, especially emotional issues.
Ecchi: Very much so; it pushes the boundary between ecchi and hentai about as far as it can be pushed without crossing over.
First Girl Wins: Subverted, possibly double subverted. The woman that Aiba is with at the end of the manga does not seem to be Kurumi, but Aiba has some sort of sexual malfunction at the end, presumably a failure to get an erection. This implies that he never got over Kurumi.
Flower Motifs: Aiba flips out when someone leaves a bouquet on Kurumi's desk, implying that she died. Additionally, in the final chapter, Aiba has sex with Kurumi as the 'sea flowers' form on the shore. Kurumi notes that the flowers are made of dead plankton.
I Want to Be a Real Man: Aiba decides to start working out and getting into other activities, partly to get his mind off his sexual frustration and partly to overcome personal issues pointed out by Kurumi.
Jerkass: Kyouko's ex, who treats her pretty much as a disposable fuck-puppet and bullies the male club members.
Ms. Fanservice: Despite the constant barrage of fanservice provided by Kurumi, in-story it's Kyouko; she is constantly wearing revealing clothing, seems to have never heard of bras, and is much less modest than Kurumi. Kurumi also never wears a bra, but it's Kyouko that makes it obvious to everyone around her.
Not What It Looks Like: The older man that's occasionally seen with Kurumi. Many of his scenes give the impression that he's sleeping with Kurumi, especially to Aiba. Turns out he's one of her doctors making house calls on her, and is very pleased that Kurumi and Aiba have found each other. He's also one of the OB, along with the head of the hospital Kurumi is being treated at.
Only Sane Man: Aiba and Katsu are very weird in ways, but they're bastions of sanity compared to the ex-president and Tatsuya.
Panty Shot: Count on multiples of these per 'collar'.
Peek-a-Bangs: Kurumi often has one eye covered by her bangs, but they're wispy enough that they only partially cover the eye...but it's not drawn on her face.
Single-Target Sexuality: Aiba becomes completely focused on Kurumi once they start hanging out with each other and Kurumi becomes a member of the Roman Club.
Snow Means Death: Collars 73-76 take place in the winter. Kurumi apparently dies the night that Aiba takes her to see the sea flowers.
Third-Person Person: Kyouko frequently speaks in third person, especially when she's excited or upbeat.
Wham Episode: The aforementioned Collar 72, in which Kurumi mysteriously disappears, leaving the Roman Club wondering what has happened and Aiba bitterly regretful that he didn't do more with her before that point. She comes back in the next collar, but her health has taken a sharp turn for the worse.
Wham Line: "Little did I know, that was the last summer I would spend with Kurumi-kun." It marks the shift from Shōnen hijinks to the actual Seinen form that the manga ultimately takes.
Played with. Aiba and Kurumi have a relationship that's fraught with sexual tension and all manner of unconventional expressions of love, but they do have an actual relationship. Additionally, Kurumi flat out says that she loves Aiba, and Aiba eventually does the same. Despite that, there's literal unresolved sexual tension, in that Kurumi controls when Aiba can orgasm.
Katsu and Kyouko qualify as a straighter example. Their relationship slowly builds over the course of the manga, and the one time when it seems like they might, a distraction gets in the way. In the final 'collar', Aiba's wife reveals that they are getting married.