can you give me an example?
When you level up a princess or progeny, one of the four stats that can be upgraded is "Lust". This is the stat that acts as their lewdness level. For princesses and certain recrutiable NPC's, at lewd level 1, they'll rarely get battlefucked, increasing their lewd level will increase the amount of times they'll willingly have sex with a combatant and increase corruption gains. Having their lewd level capped at 10 will have them have sex with combatants very frequently, like 80-90% of the time.
If you're talking about the kingdom lewdness level, it's a somewhat hidden stat that determines the salacious behaviour of the entire kingdom/map, with a higher level increasing the sexual encounters found around the kingdom and several succubi appearing around the kingdom. I believe that this is increased by frequently having sex around the kingdom among other things I can't positively confirm. A high enough kingdom lewdness level will trigger warnings and then a gameover if you don't reduce the kingdoms lewdness level in time.