I don't know if there is a manga about that but there is a Chinese Yaoi Novel about two guys pretending to be gay because they wanted to get revenge on someone, the cover does look anime like. It's only up to chapter 6 so far tho. The title is called being gay for 30 days.
Maybe this oneshot
http://www.mangago.me/read-manga/ren_ai_shinan/mf/v01/c004/
I guess you've read these two as well but I'll leave them here
http://www.mangago.me/read-manga/sensei_wa_butchozura_shite_yatte_kuru/
http://www.mangago.me/read-manga/yarichin_bitch_club/
Any manga where they pretend to be a gay couple and then, well, end up becoming gay? XD