That the young landlady was married to the old guy. The fact they spent so much time having the MC pin for her only to get torn down by the revelation that she as married to basically his sensei was terrible.

The Land lady and the Sensei thing was just yuck. No better words to describe it (it doesn't deserve even that).
It was bad for 3 reasons.
1] First, there was no fucking indication of that ANYWHERE in the manga up until that chapter. It was a literal Ass-Pull. There was nothing that even slightly indicated that those 2 were close in some special way, forget fucking married.
2] Second, an old man marrying a young girl. Now, in certain circumstances I wouldn't mind it much. But the one where he married the girl whom he brought up like his daughter? FUCK NO. That's just disgusting. The age is not the problem, it's the extreme violation of morals of having married someone who was like his daughter. That's just disgusting. Simply. Disgusting.
3] Third and what I was disturbed with the most. How the didn't even give a fuck about Kouta. That FUCKING sensei guy was LEADING HIM ON THE ENTIRE TIME. He was all like, "you have a tough hurdle ahead, bro". WHAT THE FUCK. You could've told him she was already involved or something or atleast that it was outright impossible! But no! You wanted to fucking lead the poor guy on and EVEN BEFORE he revealed everything, he actually went upto him and asked him if he liked her or not! You getting off on that?!
Even forgetting that part, AFTER they revealed everything and Kouta falls into depression AND THEY KNOW IT, they just keep flirting with each other. WTF. It's like they don't even give a shit about what happens to him and how it's all their fault!

Why can't anyone make a manga about a parent without it having a messed up element to it or screwing up what should be a very simple premise. I didn't read this expecting a love triangle or even a love story so there was no need to twist one in the first place(though covering dating while having a kid would be interesting). But even then, it was pretty obvious who it was towards, so it didn't need more twisting, at that point you're either breaking someone's necking or ripping it off.

And I'm all for doing the right thing but sometimes that's getting angry at bad behaviour, ill treatment, and unfairness. Instead of this manga wants people to be patsies, and I saw that coming too.

Aside from not telling him about his child(not ever planning to), she tries to sneakingly take her. but after such a knife twisting revelation told with no hint of even being sorry or repentance. All of this could be easily learned by her child that had gotten close to her father, and even though the child was also close to her mother in reality something like this might(and probably would) cause a rife at such crappy behaviour. But you just know it won't because this comic just picks and chooses when its going to be real and when its going to expect the audience to be stupid. Meanwhile her hardships were mostly because of her choices(though she did try to be a mother to her kids), and what she did had an insane negative impact on him and even blames him for her crap too, and even tries to push him into a horrible life. Yet he still managed to be better off(before she just marries rich) and at least have a steady job, and residence. But in terms of writing, I like her reaction towards the end and how that isn't instantly resolved. And that's basically the problem, it would take a whole other manga to resolve most of the things brought up.

In reality this would cause so many massive rifts even between the father and child, and this comic uses something too serious that it isn't equipped to deal with, and it does this repeatedly, and needlessly. I wrote in a forum post about the landlady's situation and how it made light of her bad upbringing too. The Mother(Nozomi) even puts all responsibility on a person that had nothing to do with her crap(Nitta). Despite being a semi-decent provider she gets away with awful things, and we're suppose to sympathize with her just because she's a mom? Even other characters seem to blame the loli guy despite him acting more decently than the lady. It's just insane. I'm not saying what she did isn't forgivable, it's that no one ever blames her even one bit in the first place.

It even ruins his relationship with his only possible love interest,and destroys his trust in women and (could have) potentially people as his best friend also betrayed him.

This manga is enjoyable enough when it was just about the daughter and father bonding. It's attempts to surpass that effort have a strange effect of working and working negatively. It's certainly memorable in an infamous way, and creates drama, but how necessary was that drama, and that drama is infuriating and not balanced.

The manga forces the guy to be good to the point that he can't express genuine emotion in a situation where it'd be perfectly acceptable and might be the only logical way to behave. It wants to have it both ways. And it's typical reaction comes of as disingenuous and weak.

Though his desire to be with his daughter is good, it's a little hard to believe she chose what she did. It doesn't feel fully explored but it's not vague or occult either.

The ending tries to be stereotypically sweet and the tone gets lighter but has the same mistake this series makes.

It ends on the nasty note that basically the same exact mistake gets made and it's treated like a light hearted nothing. There's the promise that things will be alright but it doesn't change the fact.

My score: evaluating this manga is difficult and confusing. My score is nuts. I think people who read these things will be better prepared than the first guys who got blindsided like we did. It's definitely flawed, I don't know if it should be commended for trying some stuff. It's hard to tell if the writing was incompetent or if the mangaka was being a magnificent bastard.