It's apparently about boys having sex with girls, but it's really about gay subtext. That's my reading, anyway.
Firstly some background, about South Korea in general and about Ji-kyun Kwak in particular. Korea isn't a gay-bashing country, but they have a big closet. That's becoming less true these days, but before 2000 there were no openly gay celebrities. Actor and male model Hong Seok-cheon came out that year, the first and and still the best-known public figure to do so, and was promptly fired from all his TV shows. Since then, other Korean celebrities have come out of the closet only to end up committing suicide after getting fired from their jobs in the same way and getting bashed online by members of the public.
As for Ji-kyun Kwak, I have no proof that he was gay, but I do know that he never married and that he committed suicide in 2010, aged 56. I don't think he killed himself because of his sexuality, mind you. He'd been a big deal in the 1980s and 1990s, being called a new wave director and winning awards for films like Long After That, Reminisce, Portrait of the Days of Youth, Walking To Heaven, Days of Roses and Deep Blue. Ironically though, Plum Blossom was a turning point and it marked the start of a decade of depression. It was his first film as writer/director since Love Triangle in 1987 and afterwards he had trouble getting his next film made. Eventually he made it and so Fly High came out in 2006 (again acting as writer/director), but that would be his last movie. He felt the movie industry had passed him by. As he wrote in his will, "As there is no work, it is very painful and hard for me."
Knowing all that, it's hard not to see this as a personal film for him. It contains:
(a) young, muscular men who take their clothes off a lot and have close friendships with each other that are never consummated but often homoerotic (possibly unintentionally).
(b) suicide for unattainable love.
(c) cinematography that's taking no noticeable interest in naked women. Notice how indifferent the camera is to their boobs, for instance, instead spending much more time on male buttocks. (In fairness though, the film doesn't actually leer at the latter.)
(d) boys who don't chase girls. Heterosexual sex in this movie is something that's instigated by women, with their male partners being uncomprehending, passive and/or fearful. It's a common pattern for a woman in this movie to offer herself to a man, only for him to walk away as if she didn't exist. There's an overwhelming gay fantasy element here. Healthy male-female relationships don't really exist. One girl (Ji-hye Yun) repeatedly throws herself at a classmate (Jung Hyun Kim), forcing him into sexual situations and then going through it all again at a later date even after he cuts her dead in the most humiliating fashion. This is because Kim is in love with his female teacher (Hee-kyung Jin), but that's a helpless, unreachable kind of worship that works almost disturbingly well as a metaphor for homosexual love in a heterosexual society. Again, there's no real relationship between the two of them. It's a brittle, idealised thing, as opposed to the closely observed portrayals of boys together.
For my money the only character here who feels like a real woman is Doona Bae, partly because she's by far the best actor in the movie. She's outstanding. You might also have seen her before, e.g. Air Doll
, Barking Dogs Never Bite
, The Host, Sympathy for Mr. Vengeance, The Ring Virus. All the cast is doing an admirable job, but Bae clearly has a quality above and beyond her co-stars. She can reach through the screen to an audience.
However even her character is the one who makes the first move, despite being a nervous virgin and the man (Rae-won Kim) being an experienced womaniser. The boys even comment on this afterwards.
(e) a visual metaphor in which our two male heroes are walking away from (and framed between) two massive breast-like hills, only for a mother-figure to walk past with a line of small children.
However that's all subtext. It's sometimes very loud subtext, especially when you've got men fainting in each other's arms and then later waking up in their best friend's laps, but nothing in the movie explicitly says "gay". On the face of it, it could almost be argued to be a counter-reading. It also captures some subtle and uncomfortable moments, so for instance there's some remarkably honest characterisation through realistic sex. We see more than one person's first time. Kim performs better than I'd have expected, to be honest, although he lets the show down afterwards. The first half-hour is slightly unnerving, with sex being pushed on people who don't want it and are unlikely to react or perform appropriately.
This is a good film, but I must admit that I'd have rated it lower without the gay subtext. That gives the film weight. However it feels honest and meaningful in its portrayal of the men, while the female characters are played by beautiful and talented actresses who've been given strong roles to play. Occasionally it's even funny, e.g. "Would you mind taking my virginity?" However I think it's fundamentally a sad film, for reasons that aren't all entirely on-screen.