Rom where you have to accept something taboo or scary or weird

I am not sure of what you're looking for but I'll try to recommend you something that can fit:

• Love Better Than Immortality (2019) (He's manipulative, uses her for his plan, and an "antihero" (villain)
• Cruel Romance (2015 (I can't even explain that)
• It's Okay to Not Be Okay (2020)  (because of psychological matters and other things I can't reveal since it's a big spoiler)
• Flower of Evil (2020)  (Same as IOTNBO)

Maybe, if you can be more precise or give examples of what you're looking for I will be able to give you the closest recs :) 

• Cheese in the Trap - questionable ML
• Watashitachi wa Douka Shiteiru - ML accused FL's mom of murder 
• Before We Get Married - affairs/cheating 

To add to those above, if I am interpreting this correctly. This is a fantastic request!  Its something I think I unconsciously look for so I'm glad you made this request. Often times, dramas make conflicts out of things that are just too overdone or not relatable or non-issues for most people today so it can be hard to take the conflict seriously. Things like noble vs commoner, dating within in-laws or those made-up hierarchy or opposing sects in fantasy dramas. Everything becomes much more engaging when it is something actually taboo to the viewers as well. 

18 Again - Ml is transformed back into the same age his children are. And his wife who doesn't know has to come to terms with being attracted to someone 20 years younger than her. (Even if he isn't that young in reality)

♥ Battle of Changsha - Quite a large age gab between fl and ml. Fl is still in school when they first meet, and he is an adult already.

♥ Do Do Sol Sol La La Sol - Again with the age gap. Fl doesn't know at first but ml is still in school when they meet. So he's just a teenager while she is in her mid to late twenties.

♥ Doctors - Student/teacher relationship.

♥ I Hear Your Voice - Again with the age gap (I'm suddenly realizing how many of these I have on my list. Hmm ...). Ml is in high school, whereas fl is an adult with a job.

♥ Mr Queen - Fl is actually a modern day man, transported back in time into a woman's body.

Movies:

A Muse - Huge age gap. Fl is still a teenager.

Innocent Thing - Student becomes obsessed with her teacher.

♥ Lust, Caution - Ml is married. And fl only starts a relationship with him because she intends to assassinate him.

♥ Method - One of the mls is married. And they start a relationship due to method acting.

My Little Bride - Student/teacher. Also arranged marriage.

Narratage - Student/teacher relationship.

♥ P to JK - Age gap. Fl is a high school student. Ml is a police officer.

The Cornered Mouse Dreams of Cheese - One of the mls basically blackmails the other into a relationship.

♥ The Handmaiden - One of the fls is trying to scam the other.

♥ Wonderful Nightmare - Fl dies when she isn't supposed to and is sent into the body of another woman temporarily, where she falls for that woman's husband.

The titles with a heart next to them, I really liked. The others I didn't so much, but you might :)

Dramas:

• Are You Human Too?: male lead is a robot
• Biscuit Teacher and Star Candy: student/teacher
• Chugakusei Nikki: student/teacher
• Garasu no Ie: age gap, female lead dates a father and then dates his son
• Gisou Furin: main female lead lies about cheating, second female lead actually cheats
• Hajimete Koi wo Shita Hi ni Yomu Hanashi: student/teacher
• Here's My Plan: female lead almost commits murder
• Hirugao: affair
• Kenja no Ai: age gap/revenge
• Lie After Lie: female lead lies in order to become her birth daughter's stepmother
• Love and Fortune: age gap
• Majo No Jouken: student/teacher
• Mask: female lead assumes a look alike's identity
• Mistress: cheating
• Mitsu no Aji: incest
• My Sister Of Eternal Flower: female lead has an intellectual disability
• N no Tame ni:  main characters are involved in a murder
• One Million Stars Falling From The Sky: major spoiler - incest
• Otto no Chinpo ga Hairani: main leads physically can't have sex. I found this one frustrating because they never solved their issue.
• Pure: female lead has an intellectual disability
• Semi Otoko: male lead is a cicada
• Someday or One Day: a 27 year old inhabits a 17 year old's body
• Starman - This Star's Love: male lead is an alien
• Sumike Sumire: female lead is an elderly woman
• Tae Pang Korn: male lead is a ghost for the majority of the series
• Unmei ni, Nita Koi: age gap
• Yoo Na's Street: female lead is a thief

Movies: 

• Air Doll: female lead is a sex doll that came to life
• Boku wa Imouto ni Koi wo Suru: incest
• Close Range Love: student/teacher
• Come On, Irene: mail order bride, abuse, sexual perversion, murder, you name it. I usually enjoy messed up movies but I didn't enjoy this one personally.
• Green Chair: age gap, very graphic scenes
• Hitsuji to Okami no Koi to Satsujin: female lead is a serial killer
• Her Granddaughter: age gap
• Laundry: male lead has an intellectual disability
• My Ordinary Love Story: warning this spoils the whole movie - male lead is a serial killer
• My Rainy Days: age gap
• My Teacher: student/teacher
• Sensei Kunshu: student/teacher
• Waiting in the Dark: male lead takes advantage of female lead's blindness to hide from the authorities in her house

 Sonme:

One Million Stars Falling From The Sky

it's japaness version? I've seen Smile has left your eyes, which is great drama, so my question is: in japaness version they are real siblings? in korean it was only doubt... and also, is japaness version very similar? I mean would I watch same drama with other actors?

 Sonme:

Yes One Million Stars Falling From The Sky is the Japanese version of The Smile Has Left Your Eyes. I haven't seen the Korean version but I can tell you about the Japanese version: Yes they are real siblings. The ending is very tragic in general. I enjoyed the Japanese version so much I decided not the watch the Korean version.

thank you, I'll try it too, and I would recommend you to try korean, you can guess from 1-2 episodes if you like it. I can tall it was one of the high quality (with amazing actors/acting, cinematography, production value, realistic details without any clieche and usual primitiveness, great chemistry) I've watched, and I've tried up to 200 dramas  ))))

 ShotaSidePart:

I've seen both versions of One Million Stars and honestly, I say go with the Japanese version. It's so much more darker and the acting and story is top notch. The Korean version is a more light/ pretty production of it and I couldn't connect with the characters at all.

glad to know, I have above good drama. I must say Korean version has some details which isn't typical for kdramas, as I can see because of Japanese original story, also I'm not surprised korean version isn't as dark and siblings theme isn't in korean version, because kdramas never have such darknes and taboo subjects (for me it's shame). of course, when you filter some important details it ruins some overall picture. though without knowing original version Smile has left your eyes with great actors and really good production is one of really good korean drama :)