I recommend My Most Fav 'Sex friends becoming Lovers' theme -> the 1st story of "Shiranu Wa Omae Bakari" by Minazuki Akira. Although some people find her art a bit weird but the stories are depicted so very Beautifully, that you'll forget it about in no time.
http://www.mangago.me/read-manga/shiranu_wa_omae_bakari/mh/v01/c001/2/
BlazingShana21's List : SEX FRIENDS BECAME LOVERS
http://www.mangago.me/home/mangalist/43227/
Here's my little collection, the ones i like. Stories like that usually had one of the guys suffers from a silent unrequited love which gives it some heart pinching elements and some had a bittersweet story http://www.mangago.me/people/tag/49344/sex+friends/
Yaoi stories with sex friends, where one falls in love with other???
(((and like he worries over it maybe or tries to win him over or something)))