Clannad is the only one I can think of, and only parts of it, like maybe Fuukos route, common route and Nagisa's route, just not the final one. Though there may be some obscene jokes containing talk of porn magazines during common route with Sunohara if I recall correctly, it's been a while since I read it though. There aren't any other japanese ones on top of my mind, symphonic rain might work but it does have some dark parts, hints towards two people having sex and somewhat complex storyline.
If you don't mind english produced one they would probably be the better choice tho they aren't usually as good, Ripples is an decent and very short one and maybe Always the Same Blue Sky, though I can't remember how the latter one went.