This ranges from getting caught seducing someone (affects only two people) to an outbreak of the Eros Plague (affects everyone). The amount that the relgion’s lust increases depends on how many people are directly involved in the event.
It must be known to the public–for example, seducing someone and not getting caught will have no effect, but seducing someone and getting caught _will_ have an effect. Every time a ruler does something lustful which is generally known, the lustfullness of that ruler’s religion will increase. This is a new mechanic that’s not visible in-game, so I’ll explain how it works. If an AI character is doing the reformation, the overall amount of lustful activity that rulers following that religion have engaged in will affect whether they pick lustful features.