I read this manga long ago and now I don't remember it's name.Manga was about popular guy X unpopular guy.For some reason they end up being sex friends.Seme is kind of cruel too...please help...

p.s they were university students if I remember right

perhaps the second story in Akanai Tobira?

It's similar but unfortunately no :/

Mimitabu no Riyuu?
You can also check this http://www.mangaupdates.com/series.html?category=Sex%2Bfriends&genre=Yaoi&exclude_genre=Ecchi_Hentai_Horror_Lolicon_Seinen_Shounen_Yuri&perpage=50
I filtered out horror, lolicon, yuri, seinen, shounen, hentai, and ecchi and put in the sex friends category

It's similat but no :/
Seme was cruel guy that only wanted to be sex friends
uke fall in love with seme and because he thought it was one-sided kind of tried to get away from him but seme got jealous

Is it Deep Flower?

Maybe... Tsumetai Taiyou ?
http://www.mangaupdates.com/series.html?id=18428

(=ヮ=)೨

or Yabai Kimochi

if there is an option between "to do" and "not to do", I will always choose the latter.

"what kind of crazy, twisted conclusion is that...?" - banana bread no puding

it's Tsumetai Taiyou..thank you very much...