FATAL has the best damage spell: Perpetual Orgasm.
It makes the target orgasm forever, which leads to a heart attack after a couple of minutes.
Tempted by comments such as yours, I delved into the F.A.T.A.L. books, expecting utter depravity and being pleasantly surprised by the autistic-level of detail where most systems are concerned. Spells, for that matter, were fairly diverse in their scope, relegated primarily to defensive or social applications and largely divorced from any sort of sexual activity.
Among some of the more interesting spells, notably those that could be implemented in a videogame, are...
Ablation
The caster is able to insert their hand into the target's body and grasp the organ they specified in the chant, forcibly removing it from the target's body with sufficient strength. The heart is a popular choice as the target is doomed to perish within a matter of minutes.
Agelast
The creature at whom the caster points while casting the spell loses the capacity to smile and any attempts to elicit one will be perceived as annoying. The target is neither angered nor depressed by the spell itself, but tends to withdraw socially as the activities in which they'd normally delight now cause irritation or boredom.
Against Menstruation
Causes the woman on whom it is used to cease menstruating after a week. The spell persists for a lunar month, after which her menstrual cycle will resume as usual.
Audio Range
The caster may recreate any memorable sound and, depending on their level, alter its volume.
Bestow Aeon
Causes the target to age, both permanently and instantly, by several million years. Aging beyond their natural lifespan is immediately lethal. Nearly all mortal creatures are fossilized by this spell.
Convert to Cannibalism
Causes the target to lose their taste for all food, actively despising it, while instilling a craving for the flesh of their own species. The target's behavior is not modified, thus some may act like animals but others are just as likely to be cunning in securing some meat.
Create Species
Casting this spell allows the creation of a mortal species as dictated by the whims of the caster. If cast by a mortal, this spell is certain to earn the disfavor and wrath of the gods.
Cup of Love
The cup of wine used in the spell is enchanted to cause its consumer to irrevocably fall in love with the caster provided they drink its contents entirely within a day.
Decortication
Causes the skin of the target to be rapidly peeled from their body, removed in up to four strips at random. Unless bandaged immediately, the target will suffer massive loss of blood. Those who survive the spell will find their sense of touch severely impaired as nerve endings are largely within the skin.
Defenestration
The target creature is forcibly ejected, by an invisible and otherwise imperceptible agent, out of the nearest window. The forces pushes them at a rate of roughly 100 feet per round. The force may be withstood by sufficiently powerful creatures, though it will not subside so long as they remain near a window.
Demokritos' Table Gimmicks
Consists of many uses, any one of which may be selected, including: to make bronze look like gold; to be able to eat garlic and not stink; to make cold food burn those who eat it; to be able to imbibe excessive alcohol and not become drunk; to get and maintain an erection on demand.
Develop Breasts
Capable of affecting any mammal, male or female, the target will immediately develop (or further mature) breasts. Males will gain fatty deposits like those of a female, not muscle, though their nipples will remain unaltered. Females who already possess breasts merely increase in cup size.
Discursivity
The target will be unable to hold a conversation without rambling from one topic to another incessantly.
Ejaculate Blood / Acid / Poison
...need I explain?
Eternal Spell of Binding a Lover
After casting, and applying the unguent to his manhood, the caster must insert the head into the orifice of his choosing, after which its owner will eternally burn with desire for repeated penetration by the caster such that they are likely to beg for his manhood daily.
Fatal
Successfully casting this spell causes all living creatures on the world in which it was cast to immediately perish. Death is inescapable, unavoidable and permanent. This spell has never been cast by a mortal (worth noting it costs an obscene amount of magic points).
Force Falsity
Causes the target to lie with every statement made verbally for the duration of the spell, despite any attempt or desire they may posses to speak the truth.
Happy Cleaver
Causes matter in the area of its casting to transmute into a sentient, self-propelled cleaver. The weapon does not require the caster to wield it, seeking out and attacking anything that may bleed within a 100 meter radius; if no such creature is present, it will attack the caster. With each successful attack the cleaver can be heard, in a hysterical fashion, emitting a chill laugh. The cleaver, for some reason, consistently displays a preference for hacking apart female dancers.
Intestinal Wreathe
Causes the target's intestines to emerge from their rectum and wind about their neck in an attempt to strangle the victim for the duration of the spell. Those who manage to survive the strangulation still require immediate medical aid and will likely suffer indefinite discomfort when defecating.
Omniscience
Grants all knowledge regarding all subjects, past, present and future, to the target instantly. The target must pass a number of skill checks, however, or suffer a large number of permanent mental illnesses. After the spell concludes, the target will immediately forget all of their omniscient knowledge and return to their normal faculties.
Perpetual Orgasm
Causes the target to endure a single, neverending orgasm whose intensity surpasses that of any other they've ever experienced. As appealing as the prospect sounds, no mortal creature is capable of normally functioning while under the effect nor can they endure for long as the unceasing pleasure causes eventual madness, muscular decay as the body twitches and convulses uncontrollably, followed by inevitable death as the heart gives out from the strain.
It also struck me that, given the erotic nature and general reputation of this system, the spell "Burning Bush" was quite distinct from what I'd anticipated. It's an allusion to the Biblical story of Moses, summoning a literal god into an actual bush. For a game with several varieties of "charm a woman into submission" or "force a woman you've kissed/touched/slept with to fall in love," there are shockingly no "give a woman chlamydia" spells.
