In ''All about lust'' one friend (Jungmin) was really supportive of her friend being gay. It doesn't match perfectly what you're asking for but it's still a good manhwa ;)
http://www.mangago.zone/read-manga/all_about_lust/?av=1
Oh, and this one!
http://www.mangago.zone/read-manga/all_night_long_muno/
Koimonogatari... it's about a guy who finds out someone is gay and is very supportive of him, but he's part of the main couple eventually. Moves kind of slowly so it gives you time to really appreciate how supportive the guy is.
hello, does anyone know yaoi stories where the main couple friends/buddies figure out that their friends are gay and, instead of rejected them, they support them? i know it is too especific, but that kind of scene warms my heart ahah ;) thank you