Warito Yokuaru Danshikouteki Ren'ai Jijou (2001)

Blurb:
Welcome to Kinsei Private School-a respected, all-boys institution where it’s rumored ninety percent of the student body is gay (or bi). For the straight Nanase, this means constantly evading day-to-day advances from his love-struck classmates. Just what is a "normal" boy to do in this sea of abnormality? Enter Heiji, who proposes a unique solution to his sempai-namely, that he and Nanase pretend to be a couple to deflect all the unwanted attention. Sounds like a good plan, except pretty soon Nanase finds himself performing all manner of embarrassing acts in order to keep up the "ruse." What’s wrong with this picture?!

Source: www.anisearch.com/manga/316