I generally hate NTR, but probably for a slightly different reason than others. I loved this one, because even though it is technically NTR, but the feelings involved make it more of a love triangle. It doesn't even matter that in the beginning of the manga she already was going out with her boyfriend for 5 years, it can still be love triangle. Even though it is not exactly romantic, it is about doing everything for the loved ones though.
Also I see many people raging after reading this, but I instead find it bittersweet. I kinda can see why they rage, but I think what they don't understand or don't want to understand is virtue of selflessness. Because certain selfless actions hurt person in question, but you know that is what it take to put other people's happiness over your own, even if you do it the wrong way...
More specifically on selflessness, spoiler ahead:
Yes she broke up with both of them, but only because she couldn't find a way for everyone to be happy.
She even had an affair because she thought about her best friend happiness, even though that best friend had an affair with her boyfriend.
She does those things one sided, she tries not to show her pain, because she wants to help others, but doesn't want others to be forced to help her. Which results in a lot of contradictions because when both want the other to be happy but can't think of any other way then actually breaking up.
Well sure threesome part was weird in some way, but hey I actually liked this unorthodox approach to relationship. And it is truly about both guys trying their best to make her happy. Which is also selfless in it's own way.
I also saw that some people didn't like the ending, but I had them shipped from chapter 1 myself, so I have no problem. Even after seeing the side of her 5 years boyfriend it got only reinforced for me, because he looked more like a guy who took advantage of naive inexperienced girl than actually a love interest. While she actually ends up with the person she truly loves no strings attached and who loves her back in the same way.