I don't understand why everybody is so angry with Kenshi's reaction, to me is pretty under...

Here are some reasons:
He lied!
He kissed another person
He missed nights from home
He never talked with Shingo
He never bothered to explain anything to him
No respect whatsoever
He came home (in the morning) with another man smell on him
He decided on his own to leave without his bf for an year or so.
And with another man! And please don't tell me that is for work. You don't kiss another man for work....
And after the Shingo cast his pride and he goes after him.... What keiichi do?

I'm not saying Kenshi is in the right, after all he has done all those things. But if something Shingo has done some of those to:
He never talks with Kenshi, He doesn't bother to explain anything, he has always been insecure about his relationship with kenshi and he also decide thing on his own, like pretending to know his partner feelings without asking.

So while I understand that Shingo has been hurt the most, I'm not going to forget that Shingo is also guilty, he should have talk about his insecurities, i'm not saying that the "no talking" is an excuse for cheating, but that silence has helped to damage their relationship.
Now you can not expect that after two persons break up "unilaterally" (because Shingo was the one who wanted to break up) and without actually talking, the other side is happy to see him, and open his arms with a smile.
So of course he gonna be acting cold, and I'm no implying that he has the right to it, at least not after that kiss, but that a normal reaction.

I blame both of them for what had happen, to me somehow they are equally guilty.

I can't blame Shingo... and i think this is way past his insecuritis...
Dosen't matter how sure you are, if someone did to you all that Keiichi did to Shingo you will become unsure! To be onest, i will want to break up if someone will do something like that to me.

Although I respect your opinion, I can agree in this particular case with it. Because to me in this very occasion Both of them could have make everything easy if they had just talk. So Kenshi IS IN THE WRONG i would never denied that, but Shingo only keep everything to himself, and that to me is also wrong. He just made assumptions (Some right, some wrong), and rash decisions, that as we can see he finally regretted.
Talking is like a basic in a relationship and I can't believe the author is addressing to this until now. they had always had this problem (to me at least)
So Both of them are guilty, and while the don't share the same 'amount' of guilt, they could have acted in a better way.

I agree on ur thought. A relationship both parties have to work for it to actually work out. One person can’t do it alone. Both Kagami and Shingo has their faults in this situation and they could have tried better to solve their problems but this is a story so, but we can’t just blame just one person here