Addicted Heroin - Strongly implied they sleep together and if you watch uncut version there are some steamy kisses.
Cruel City - This one isn't romance focused but there is a sex scene in it that is unusually steamy for a kdrama. It is however quite sad, so if that's a dealbreaker you might want to skip this one.
Dating in the Kitchen - Some steamy kisses and implied sex.
Make Our Days Count - Quite steamy but does end sadly. You can however skip the last 10 minutes or so and get a satisfying ending anyway.
Trapped - Very steamy kisses and heavily implied sex.
Long Time No See - About as steamy as it gets honestly
Mr Queen - Steamy kisses and off-screen sex.
Innocent Man - Steamy kisses (don't remember if there is implied sex, but i think so).
The King's Woman - This one does end sad. But the couple has really great chemistry. Though their romance is fucked up
Scum's Wish - Very steamy kisses. And some characters do have sex.
Tomorrow With You - They do the do