You place a curse on the subject.
Book of Erotic Fantasy
Author: Gwendolyn F.M. Kestrel
Level: Cleric 3, Sorcerer, Wizard 4
Components: V, S, M
Casting Time: 1 standard action
Range: Touch (see text)
Target: Creature touched (see text)
Saving Throw: Will half
Spell Resistance: Yes
If the target attempts sex with any creature other than yourself, she experiences wracking pain causing 1d6 points of damage per caster level (maximum 10d6), with a Will save halving the damage (rounding down).
If you have placed a divine or arcane mark upon the creature, you can cast the spell at any range.
Material Component: A drop of sexual fluid from the target creature.