Don't believe anyone who calls this trash. It's a powerful but also surprisingly subtly written drama that finds all kinds of unusual angles on teenagers, sex and relationships. There's a lot to admire in how it portrays all kinds of moments and plot beats. Rui's matter-of-fact appraisal of losing her virginity, for instance. The extreme shades of grey in both "good" and "bad" characters. The way near-sex scenes will so clearly be about characterisation and ulterior motives.
At the same time, though, these people are idiots. All of them. Every last one. I'd often find the show agonising, because I'd be going NO NO NO STOP IT. No one talks to anyone, except at gunpoint. (I exaggerate, but not by much when it comes to the important stuff.) Natsuo's admirable in the way he's not ruled by his penis, but less so in his determination to keep things secret, either pointlessly or damagingly.
1. We're siblings now! Don't tell anyone.
2. We're in love with each other, seeing each other regularly and talking about sex. Don't tell anyone, including the person who absolutely has a right to know. That was shitty, Natsuo.
3. Who knows, maybe the school might have been okay with a certain development if they'd been open about it instead of letting it get found out dramatically? That's speculative, admittedly, but they'd been keeping that quiet too.
In short, the biggest problem with these people is not telling things. One of the best things about the damaged, sweet and promiscuous Momo Kashiwabara is that she's blind to all that. Mind you, she's suffering from the opposite problem, i.e. her name is mud with boys and girls, and everyone knows about her.
The show has what sounds like a hentai premise. Natsuo's in love with his teacher, Hina, but then one day he ends up having sex with a girl (Rui) at a group date. (Rui's cold-blooded about it and had mostly just wanted to know what it was like.) Then, later, he discovers that his widowed father is getting married to Hina and Rui's mother! They're going to be siblings and they'll all be living together! Goodness me, what could possibly happen?
Answer: melodrama, with assorted reasons why people shouldn't have sex. Some are good (teacher-student) and some are nonsense that's being taken too seriously (you're not real siblings).
(Lovably but maddeningly, the middle-aged parents keep wondering if the current latest shenanigans are because their children disapproved of the remarriage. At one point, they nearly divorce because of it. You'll want to shake them out of this belief that everything's about them... but it's also yet another damaging instance of gratuitous communication failure.)
The show's funny, but the funniest episode (ep.10) is also the most STOP IT STOP IT I CAN'T TAKE IT. There's a stretch in the middle where I wondered if the show was drifting into a harem set-up, with four girls who'd all be good matches for Natsuo in different ways. But that's not what it's doing. English-speaking Alex can be annoying, but he's meant to be. (A little, alongside his good qualities.)
Lurid? Sometimes, yes. Idiot main characters who need help and understand absolutely nothing, e.g. tact, delicacy and/or the importance of talking to people? Yes, yes, a thousand times yes. They'll try your patience, but it's a roller-coaster ride with them. Great title sequence? Wow, yes, especially the theme song, by singer-songwriter Minami. There's not much I'd change here, although I wish they'd given Kashiwabara more of the spotlight later in the show. It's absolutely not what I'd have written, but that's mostly because I don't think I'd have been capable of it. You'd need to go somewhere special as a writer to plumb these depths. I admire it, but it made me suffer.