For a version before the old sex system was removed, just about any sex act boosted it a little. For example, I would just spam kiss a few days in a row to max a slave's charm for public entertainer.
For a version after it was removed, I can't help. My laptop is so old and messed up, that I can't play the newest version without buying a new one. I'm thinking about it quite hard...I miss being able to play games and watch YouTube without the threat of my computer suddenly shutting itself off... Minecraft, oh how I miss thee.