Basically, it turns out that the protagonist had some rare disease that would kill him before he turns 16. The girl then makes a deal in secret to have him treated and saved in exchange for signing up to get married to a certain someone, and she also has to agree to never mention this deal to anyone.
Now, one thing I want to make clear here is that the deal itself is actually pretty interesting, and I would've thought it was a cool twist if they handled it well enough. The problem is, the disease is one that doesn't exist in real life; it was completely made up for the sake of using it as a plot device, effectively turning this story into a fantasy rather than one grounded in reality. That, in turn, makes all the characters seem more like artificial constructs than actual people, making it harder to symphathize with them.
They could've easily kept the premise of the deal but made it about the protagonist needing a heart transplant or something, but the fact that they decided to go with a ridiculous, fictional disease really ruins my overall impression of this manga.

The secret here is not so absurd one, the thing here is that she gets her hands over some info regarding yukari that is with rapid development in the genes and DNA field in science they were able to predict who will die before reaching the age of 16. so yukari will be having that genome in his gene so in order to protect him(the first love her life), she decides to do what ever she could.
And to her advantage as well as to her misfortune the govt officials approaches her saying that her genes are suitable for govt special notice and if she accepts it one her wishes will be fulfilled at any cost, but it comes with a price of her marrying some one who is too older than her.To which she accepts that condition, to save yukari.
And the party involved in the medical treatment regarding this terminal condition(that is yukari) must not be told of any of this or else his secret treatment will be annuled or dissolved.So she struggles and suffocate within her to let go of her first and most dearly love yukari.
I must suggest one to read the complete manga after done with watching the anime the manga is still is ongoing but the plot is getting interesting with the each chapter and it is also said that the manga will hit it's end soon if no disturbances were to occur.
Well there is still so much more I want to say regarding the story but I only felt like revealing the main and most important spoiler of the actual story.

I am completely caught up on the manga, it's pretty good, but I have a few things to say:

Nejima is a completely generic main character, and I don't get how all 3 other side characters fell in love with him (Misaki, Ririna, and Nisaka). Theres nothing interesting about him whatsoever and he's actually kinda annoying because all he talks about is kofun and Misaki. He's also REALLY DENSE, because he doesn't feel that Nisaka, Misaki, and Ririna like him. I that he talks about kissing and loving Misaki IN FRONT of Ririna when it's obvious she likes him and her feelings are getting hurt everytime. Also, Ririna is more decent than secretive Misaki. I don't get why she hides all this crap from all her friends, including her "boyfriend" Nejima. It's so stupid because all she does is lie to everyone and keep secrets.

I am only continuing to read this manga because I ship Nisaka and Nejima. It's the only decent ship and its soooo cute. I think both Misaki and Ririna are very annoying, so I don't wan Neji to end up with any of those girls. Unlike Misaki and Ririna, Nisaka actually confessed his feelings and wasn't being a chicken, even when people might make fun of him.
(Nisaka is actually suuupperr hot. If anyone of you disagree, report this comment. I'll wait.....)
So I was literally fangirling my voice out when he confessed, but I think it was actually super retarded that Nejima KISSED HIM, even though he's STRAIGHT AND DOESN'T ACTUALLY HAVE FEELINGS FOR HIM.
And that's my Ted Talk.

Yukari marriage huh? I wish this thing will never happen in our real life, because we dont know how chaos our society will be with this kind of program. This manga author is trying to be fair with us, she never make both parties unhappy, I mean for people who rooting for Nejima X Misaki, there is still hope, but it looks really hard, I mean how the hell they can annul the notice. Its like they need like 100++ chapter to settle everything, cleanly. For Nejima X Ririna, looks like there is bright hope, with both keep develop each others feeling bit by bit, its quite unfair because its basic in human psychology study where male and female will automatically attracted to each other if they keep talking and meeting for a lot of time. Pity for Misaki fans but for now there is still a long way to go. I quite hate Misaki character because he looks like he want to give up his love & just want to go along with the government notice. He is a weak male lead by the way. But yeah, keep reading....

Ch 74-76 is the most beautiful confession (not the usual confession setup in manga) I've ever heard. After chapters of wondering what Takasaki thinks (or if she even cares!), we can finally hear it straight from her. After that, the chapters with Igarashi Shuu in them (around chapter 83) will slowly reveal more of the mysteries. Chapter 106-107 on Arisa Mimasaka's love and friendship can get quite deep too.

I was suspicious of Misaki Takasaki 1) right after Yukari Nejima's confession and 2) when she was avoiding him. However, after Takasaki re-confessed in chapter 75 and Igarashi talked about Takasaki's love in chapter 97-98, those suspicions flipped. We'll see in later chapters how more info will be revealed.

Before I get started, there are several spoilers, so if you want to jump into this manga blindly don't read.

So this story is set in a time where the Government assigns you a marriage partner based on their "science." They claim a near 100% happiness rating with their pairings that are based partially on genetics and partially on data collected from every person from the time they're born until they're 15. Apparently, people are even able to choose if they accept the pairing or not with no consequences! Except if they turn the offer down it'll be a permanent strike against them and people that do get divorced even with this "perfect" get ostracized.

What we have here is the perfect mixture of eugenics programs mixed with government control, all tightly bundled in a package that says "for the greater good." They sugar coat the system, but do their damned hardest to get these people together, going as far as dragging them into a classroom to watch porn as an "educational film" and then telling them all to go to some private rooms to get some hands on experience.

Then there's the general hopelessness of the cast. A lot of people just accept their spouses because apparently the government can do no wrong in this setting, even though the entire system is rife for abuse. The government workers claim they don't take looks into account when pairing people, for example, but the only ones who would know that are the people running the system - the very people who would abuse it like nobles in the olden days that would steal the wives of commoners.

Even then I'll still read this since I want to see what kind of wreck this train is heading for, though my suspicions suggest it will end with a "see the government was right!"