I really want to read it again, but not remember what it called.

So, it start with a popular boy A who receive a love letter from a girl, he come to the back of the school and said "Sorry, I can't go out with you" without looking at the person face. But of course, it's not the girl but another popular boy B. B has never been refuse before, so he got angry and have sex with A, then asking what's A name.

This time it's A who got angry, so he went away.
B is a strange guy, he doesn't remember the person's face, but only the body. So he go around kissing other people to remember who A was.

Can someone help me to find it again?

I'm pretty sure you're talking about Koishiteru!?

Yessssss!!!!! Thank you so much! <3
\(^.^)/ You really save my day.