depends on the character, some scenes can be unlocked purely on high Love Points (LP) which is the affection, some require Kink Points (KP)
The surefire way to get a sex scene to to get a 100% rating on a date with that character.
Otherwise, the date scenes can also be done via Booty Call from contacts but usually the girl needs a decently high KP to do it (LP in some cases) or they get mad. I would save then try booty call around 20:00 or so, if they accept, then good, if they don't then go back a save
also you can download the walkthrough from the Development log on this page, it tells you the requirements for each sex scene.