I'm pretty certain that is for the lust battle style. They have three styles melee, ranged, and lust. The whip and cum jar are for a lust fighting style.
The cum jar works fine as an HP restoring item in other battles, or outside battles. But in certain battles like the grauls, Bradley, and the angry thing, it triggers an attempt to seduce with zero lust damage and zero healing. It's a bug.
