[marialite (Kannazuki)] Koutensei Seitenkanshou no Karesi

Yep, just gave it a full read and Dyh is right. Really interesting plot.

No as a Japanese learner I can confirm that the light hair gal (the MC) is actually a dude suffering from a kind of reversible "TG disease", which has become an advanced one, causing him being stuck as girl most of time and soon he will never turn back again. He and the heroine are childhood friends and have a crush on each other, but they didn't confess their love (what worse, heroine doesn't know the gal is MC). MC wants to give up and get brainwashed (government service), totally forget he was ever a man, but the health doctor encourages MC to confess the truth and his love to the girl, before it was too late. To his surprise, the heroine accepts him and is revealed to be bisexual. MC turns back to man and have a sex with heroine for the ever last time. In the end, they become a lesbian couple.

Girl finds out she's les but her friend isn't. Turns into dude to date straight friend. You don't get too many FTM gender bender.

englishhhhh

This one is interesting, needs a translation tho