Garasu no Ie : a woman marries a single father who lives with his two sons, both adults already. One of the son falls in love with her... Quite a sensual drama.
Sore wa, Totsuzen, Arashi no you ni... (2004): Housewife & teenage boy
Yuriko no Aroma (Japanese movie, 2010): Adult woman & teenage boy

...and the K-drama Big (2012):  Teacher & high school boy ( though the "boy" is played by Gong Yoo for most of the drama so...)
whys is it no one recommends doctors?  its a teacher/student love story :)
Meun Kon La Faak Fah - http://mydramalist.com/20040-muean-khon-la-fark-fah   They are step siblings. There is also one sided love between adoptive siblings as side story
Secret (Kdrama) not sure if fits taboo, but the male lead girlfriend dies, the female lead takes the blame. Later they develope romance, taboo or not, it will be pretty weird in real life. 
I would personally not put gay romance into TABOO category unless it is about minors and/or incest.


'Ani ni aisaresugite kommatemasu' (Japan) (4 episode drama and then newly released movie) brother/sister love
'P to JK' (Japan movie) 16 year old girl marries a 26(?) year old cop
'Kimi wa Petto' (Japan drama) (also 'You're My Pet' Korean movie) older woman/younger man (she is keeping him as a pet)
Witch's Romance (Korea drama) 39(?)year old woman and 25 year old man (It really isn't that taboo to me but it was a good drama with Park Seo Joon and they made a big deal out of the age gap in the drama)
Watch:

Sealed with a Kiss - Chinese drama with indirect themes of Dominance & submission (it's an alternative lifestyle). I don't know that the producers consciously set out to make a D/s drama, but that's what it essentially became based on how the relationship between the leads manifested itself. I always say - when rec'ing this drama to others - it's a drama for mature audiences only. If you're too young, with no life experience you probably will not enjoy (or understand) this drama much. You also will not enjoy this drama if you're one of those women who don't like dominant, somewhat controlling men.

My Man - Japanese movie, about an older man having relations with ... I can't remember his relation to the female lead. I think she was related to him in some way ... uncle or something, and she's also a high school student whilst he's in his mid - to - late 40s. More than the age gap in and of itself being taboo, it was how the protagonists presented their relationship to the small fishing village that they lived in that was taboo. Japan is still quite a conservative society, despite a nefarious underbelly, but these two were so 'shameless', esp given that there was some familial relation between them. Anyway, I won't say more than that so just watch it and see for yourself.

I don't actively seek out content with taboo themes, and will even tend to avoid them if they try to push the envelope too much ..., but I have to say that is picture was stellarly made. The plot was pretty good, but what I appreciated the most was the filmmaking ... how well everything juxtaposed with each other ... the bleak cinematographic colour palette matched the bleak lives and setting of the protagonists. The best cinematic experience for me is when all elements -- both the artistic and the technical -- come together nicely to create an excellent movie. This might not matter to you as much, but trust me when I say it does enhance the experience of watching films. It's the difference between a film that's just for entertainment & a film that is both entertainment AND art.
Devil Beside You - step-brother/step-sister relationship
Himitsu no Kankei - Sensei wa Doukyonin - student/teacher relationship; surprisingly, it had a satisfying ending
Mr. Back - the male lead is technically 70 years old and he falls in love with a 30 year old woman

 Ansuya:
Massaya 1999  - http://mydramalist.com/13492-massaya MCs are cousins.

Blood-related?