I wonder when will kenji stop being an asshole and start being honest Because the seme de...

I agree. Most times he showed no love or affection to him, went on dates with other men, and generally was very mean to his “lover”.

Guess he's afraid to fall in love again and rather have one night stands / sleep around

Stop? That's just how he is, and Yoshito fell in love with just that.
I personally love how Kenji's trampling all over him, Yoshito's such a dumbass, lol.

You love it when he is trampling all over him?
Should I say that's incredibly selfish and mean?

Yes.
You say what you will.
I think you're kind of missing the point of his character, though. Kenji has a need to stand on top and look down on someone, but also to be pampered and looked after. Yoshito fulfils that role, and does so happily. He actually enjoys that treatment, but as things are now he would just appreciate a liiiittle more acknowledgement and affection.
It's not fair to look at only Kenji's flaws and where he crosses the line, either. Yoshito can be incredibly annoying and overbearing. Personally I couldn't stand such a person whatsoever, but Kenji actually does to some degree.

If he is so annoying and somehow his is as Grand as kenji's insensitivness maybe he should reject him completely instead of leading him on and giving him hope

Flaws *

Well, he obviously like him enough not to do just that.
On the other hand, if Yoshito feels that the trade-off just isn't worth it he is free to give up at any time.