Depravity stat affect RNPC clothes size as well as their "activity" in game, you might have seen RNPCs act lewdly in public, if you play by default with small or no customization on RNPC this will happen by form a hug, kiss, etc.
The higher the stat will also higher the chance for that NPC to gain "Pervert" Attribute, for example if you set NPC Depravity stat to the max which is 120, it will guranteed that NPC is a Pervert.
Another effect is, it's easier to do lewd-thing with them.