Everything else is pretty adequate, lore-wise, but how does a charming spell get transmitted through bodily fluids? It's supposed to be a sort of mental control without any physical changes, no?
It's magic. I ain't gotta explain shit. But i will, because i am as much a pervert for details as for anything else.
Charm magic is dangerous because it's self-reinforcing. The longer one remains under the effects of the Charm spell, the stronger it grows, untill it eventually developes into the Dominate spell. When used on humanoids, or non-monstrous beings, This is well and fine, though frowned upon expect for some few niche cases. But when it comes to monsters, the far more complex spells, Dominate, or Charm Monster are required for continued obedience. If the Charm spell is used on monsters, it will continue it's self-reinforcing loop near indefinetly. This eventually results the spell becoming too powerful for the caster to control, and the monster breaks free of the casters control. As the spell will supersaturate the biology of the victim as it continues to grow in strength, it becomes transmitable through their bodily fluids, thus resulting in a rare case of a spell 'overwhelming' the caster.