Holy crap this is a good manga. Why didn't I read it until now...
The theme of declining birth rates leading to government intervention in coupling is nothing new in manga, of course. Usually this leads to some optimistic, sugary romantic comedy fluff. Not so, here. Sure there still are romantic comedy aspects, but under the surface is something much more sinister. This is a world in which birth rates declined so dramatically the few children that were born (the "last gen") are treated like some exotic endangered species. The protagonists here have been placed into this Showa era-inspired but obviously artificial environment and mostly left to their own devices, but there's also some real Orwellian monitoring going on. And as we see in chapters 7-8, there are real consequences to stepping too far out of line here. In fact, some of the content is sufficiently dark I'm surprised this was published in a shounen magazine.
The main protagonist is kind of interesting. At first he appears a tad dim-witted, but later on it's revealed that he's more aware of what's really going on than he lets on at first. It's just that he understands that resisting the adults is mostly futile so he goes along with things and saves his strength for when it really matters. This actually makes him more effective than the fiery Sakurako, who is always lashing out but accomplishes little.
There's also a bit of a mystery aspect to this story. Many things are teased at, but the reader is giving no clear answers.
Why was there such a drastic reduction in fertility in the first place?
What are the New Gen? They're not normal humans.
Why is there so much interest in Hanasaku Haru specifically? What happened in his past.
A lot of mangas hit you over the head with their settings but not so here. Things are only revealed gradually and a lot of it is show not tell. The reader actually has to put the pieces together himself. It's a fascinating world with some solid characters that really draws you in.
And then there are the occasional ecchi romantic comedy moments as well, but they actually kind of make sense here when you realize what the whole point of this facility is...