FL is already in a relationship but falls for ML

One spring night

Before we got married

Love at night

^^both are exactly like you want ML tries to woo the FL while she is in relationship with another man and has good chemistry.

Some what similar ones:-

Discovery of romance (it's more like ex bf coming back but she's in relationship with another man)

The spies who loved me (it's ex husband that comes back while she is remarried and it's more of a spy thriller than a rom-com)

 iDRAMA:

romance dramas where the female lead is already in a relationship but then she meets the male lead and they fall for each other

• +1 One Spring Night
• +1 Before We Get Married
• The Crowned Clown
• Ms. Perfect
• On The Way To The Airport

Close but not quite:

• Empress Ki - The FL meets ML 1 (a king) first and then ML 2 (an emperor). Both ML 1 and ML 2 develop an interest in the FL early on. The FL gets into a relationship with ML 1 after meeting ML 2 for the first time (iirc), but she is then forced to be a tribute / slave / concubine to ML 2. She hates ML 2 at first, but then she also comes to love him.
• The King's Woman - The ML and FL met when they were kids. The ML fell for the FL but they were separated. The FL grows up and has a relationship with another man. When the ML finds her, he forces her to abandon her relationship and basically become his hostage. She develops a complicated love / hate relationship with the ML.  
• The Imperial Doctress - The FL meets the ML (the emperor) and the SML (his half brother iirc) around the same time and becomes friends with both, and they both fall for her. The ML is married (a political marriage), and doesn't reveal his identity to the FL. The SML proposes to the FL and she agrees to marry him. The SML turns out to be a pretty awful husband / person, and the FL and ML go through some serious trials together. It takes a very long time, but the ML and FL end up together in the very end.
• The Story of Ming Lan - The ML and FL met as children, but the ML loses his way for a bit and makes some bad choices. By the time he starts to figure things out and realizes he wants to marry the FL, she's already pretty close to getting engaged to the SML. When the SML fails to fight for the FL, the ML implements a plan to get engaged to her himself (since it's a historical drama, it's mainly her father's decision). The ML doesn't really woo the FL until after his plan succeeds. 
• Witch's Romance - The FL was engaged, but her fiance disappeared before the wedding. Years later, the FL and ML meet and (despite their age gap) fall for each other. Just when they're about to get together, the FL's fiance reappears. 
• Kimi Wa Petto - The FL breaks up with her boyfriend immediately before meeting the ML. Later on she gets into a relationship with new guy while simultaneously keeping the ML in her home as her "pet".
• Moon Lovers: Scarlet Heart Ryeo - The FL meets the SML and then the ML (who are princes and half brothers). She falls for the SML (who is married) while his wife is dying and starts a relationship with him after his wife dies. The SML fails to stand by the FL in a time of hardship, but the ML steps up and the FL eventually gets with the ML instead. He also turns out to be a disappointment, and she ends up leaving him too. 
• My P.S. Partner - K movie.

 Veera94:

Before we got married

Love at night

^^both are exactly like you want ML tries to woo the FL while she is in relationship with another man and has good chemistry.

Some what similar ones:-

Discovery of romance (it's more like ex bf coming back but she's in relationship with another man)

The spies who loved me (it's ex husband that comes back while she is remarried and it's more of a spy thriller than a rom-com)

Thank you

 maidenholmes:

Nice Witch (not a fantasy despite the title) A not-very-happily-married identical twin takes her comatose sister's place as a stewardess. Her mean sister wants a handsome pilot - he likes the nice sister but can't stand the real one.  The FL is in something of a quandry since she is attracted to the pilot, but she doesn't want to be disloyal to either sister or husband. Cliched, yes, but fairly entertaining.

Th

 maidenholmes:

Nice Witch (not a fantasy despite the title) A not-very-happily-married identical twin takes her comatose sister's place as a stewardess. Her mean sister wants a handsome pilot - he likes the nice sister but can't stand the real one.  The FL is in something of a quandry since she is attracted to the pilot, but she doesn't want to be disloyal to either sister or husband. Cliched, yes, but fairly entertaining.

Thank you

my ambulance : thai drama  not exactly cheating but fl has a magical power of teleportation.  aka ML can teleport to wherever she is when ever she needs him to.  they have been dating for 15 years. and this magic only happens to the guy she falls for. appears SML, whom doesn't even know fl and vice versa and suddenly he can teleport too. kinda childish power but if you look from metaphoric perspective then ya can like it