Rom where leads struggle with morality of their relationship

• Sassy Go Go - The leads become step siblings.
• Something in the Rain - The FL's family treats her best friend and her best friend's brother (the ML) like part of their family.
• The Princess Wei Young - The ML is a prince and the grandson of the Emperor of Northern Wei. The FL is a princess of Northern Liang, a separate kingdom that paid tribute to Northern Wei. The FL's family was wiped out by a general of Northern Wei and framed for rebelling against Northern Wei. Northern Liang was subsequently conquered by Northern Wei, and its people were made into slaves. The FL blames the Emperor of Northern Wei, and when she learns the ML's identity (that he is the Emperor's grandson) she feels that she cannot be with him.  
• Mr. Queen - ML 1 (JBH) identifies as a straight , cis man in modern times, but his soul ends up in the body of KSY a straight, cis woman engaged to the King. JBH marries the King as KSY and struggles with the fact that he is attracted to / falls for the King. KSY is the daughter of the most powerful political family in the kingdom / the King's biggest rival. He is suspicious of JBH (in KSY's body) and considers KSY his enemy because of KSY's affiliation with the Kim family. He struggles with his feelings and is in denial for a while, but ends up falling for "her" completely. 
• I Hear Your Voice - Age difference (noona).
• The Imperial Doctress - ML (the Emperor) is married when he meets the FL. She gets engaged to his brother and later marries another man who holds her, the ML and some of their men prisoner. 
• The Smile Has Left Your Eyes - The FL's adoptive brother disapproves of her being with the ML. The ML is later falsely led to believe that the FL is his sister.
• Lost Love in Times - There is a prophecy involving the leads (a prince and a sorceress): dual stars, great change. To prevent a calamity from occurring, the royal family is forbidden from having ties with sorcerers. The ML and FL get together anyway, and a tragedy occurs on the day of their wedding. The FL has the ability to enter a different timeline where the ML has not met her, and there she tries to avoid a relationship with the ML because she believes a tragedy may occur again if they are together.
• Come and Hug Me - The ML's father killed the FL's parents and almost killed her too. For obvious reasons, the ML feels guilty / unworthy / sorry towards the FL even though he had no responsibility for what happened.
• Healer - The ML works as a man for hire and lives isolated from society. After his mentor / sort-of guardian is killed, he believes the FL would be in danger if she was with him and withdraws from her.  
• Witch's Romance - Age difference (noona). The ML is also getting over the recent death of his fiancee. 

Of course number one of immoral relationships are The Smile Has Left Your Eyes (2018) and it's orginal Japanese version Sora Kara Furu Ichioku no Hoshi (2002). But they are not comedies.

Also Hotel King ML thinks FL is his sister (there are some comedic parts).

And That Winter, The Wind Blows  ML pretends to be FL's long lost brother. It's a great drama and I recommend you to watch it. It's not a comedy though.

Maybe Irreplaceable Love?

 emaureeng:

Nine: Nine Times Time Travel - it's one of those butterfly effect things. 

With many of these titles, isn't the situation potentially awkward rather than immoral? If the relationship doesn't work out, their families have to deal with the fall-out. In Nine, the time travel-adjusted relationship was considered immoral. However, in North America, it certainly wouldn't be.

Also, in dramas I've often heard references to how marriages might look in a family register, but I have no idea what a family register is.

A family register is a booklet every Chinese citizen has, that records their place of birth, parents, grandparents, spouses, place of residence, marriages, divorces, kids...

When you get married in Europe you also have go to a government bureau and ask for your register, where they make sure you are not already married and are not related by blood. In China they just have all that in a handy booklet they carry around with them. 

But while in Europe you can only access this register yourself and the future spouse just gets a message that says Okay or Not Okay to get married, in China obviously a future spouse (and their family) could see all the taboo, shameful stuff in your family.