I've seen almost all the dramas AH recommended so, I'll add to that list.
Eternal Love Aka Ten Miles of peach Blossoms- The ML is arrow straight on his love for the FL. Even after thinking she's dead for over 300 years he stays true to her until they meet again
5-Ji Kara 9-Ji Made- It's love at first sight for ML, she's not interested at first but he pursues her and she falls in love.
Attention, Love!- ML is slow to recognize his feelings but she's his only love. No second FL in sight.
Beautiful Gong Shim- He falls in love with her early on and is very cute about it. She's not interested at first, she likes the second guy. Is funny how the ML gets jealous of the second ML.
Boss and Me- There's a second FL but ML has no interest in her.
Blood- Vampire romance, cute couple. They married in real life because of this drama, that's how great the chemistry was.
Doctors- They fall in love early on but he's older and she's his student. They reunite when she's an adult and start the romance.
Fight for my Way- It takes them some time to realize their love because they are childhood friends.
Goblin- She's his fated bride.
Ho Goo's Love- Cute puppy love, he's been in love with her since high school. Warning: There's a rape story line that might be triggering for some people. Nothing is shown though, just the discussion of it.
Introverted Boss- He's very shy but it's cute. The second FL is his sister, I don't remember any other FL.
Kill Me, Heal Me- She's a psychiatrist, he's in need of one...boom love.
Lost Love in Times- He falls in love with her, and only her, twice.
MARS- A little darker but the romance is cute and he only loves her. There's a b*tch that comes into the picture for a couple of episodes but she goes away quickly.
Moon Embracing the Sun- This dude is the embodiment of faithfulness. He believes she's dead for eight years. And, even though he was forced to marry the SFL, he never touches her, even with all the pressure to produce a heir since he's the king. He emphasizes this to the female lead lol how he had to exercise to get rid of his "male desires", is really cute.
My Amazing Boyfriend- Supernatural love story.
Oh My Venus- The SFL is not even in love with the ML, so there's no competition for the FL.
Pinocchio- He falls in love with her since he was young, they grow up together.
Reply 1997 and Reply 1994- You actually have to wait to know who is the real ML but in both dramas both leads love the FL unconditionally.
Secret Garden- He falls in love and is a little weird about it, but he's dedicated.
Shopping King Louie- Puppy love, very cute.
The Legend of the Blue Sea- Re-incarnation plot, they fall in love in both lives.