X is Hijacked Reproduction?

Different people have different things that set them off. The same is true of gamers. If your desire is to disturb, frighten, or weird out your players you need to have a broad arsenal at your fingertips. Common things that can set players on edge include (but are certainly not limited to) scenes or implications of: torture, cannibalism, graphic violence, mutation or body dysmorphia, various forms of aberrant sexual behavior (incest, bestiality, necrophilia), or even just particularly creepy monsters (giant insects, things with tentacles). Some players though, are completely immune to all such attempts--maybe their culture does not have sexual hangups, maybe they think humans are just meat, or maybe they are inured to violence from playing D&D for years. In these cases you need something different...

My players largely fall into the later category. They are unflappable in the face of typical Raggian horror. All of our games feature normalized images of cannibalism and all manner of sexual interactions, my players embrace random mutation, I allow horrible tentacle-monsters as PCs, and violence and torture are standard practices for all murder-hobos. So what is a DM who wants to weird-out his players to do?

Well, all the other things aside, it turns out there is one trigger still left to my jaded, 30-something, male players: the biological imperative.

The what, you say?

The Biological Imperative, the need to survive and reproduce. Reproduction is often overlooked in high-fantasy adventure games, but, eventually, even your murder-hobo wizard is going to want to settle down and pass on his genes. If your players are all in the middle-aged, settling down and having babies phase, then this is something they (consciously or not) are pretty much always thinking about. So, if we run a game where characters can, and do, reproduce (even if it is only an implied thing and not something that actually happens to the PCs), how can we abuse that to make our weird-fantasy game weirder?

Xenogenesis. 

Xenogeneis is the process by which the offspring produced by a coupling are completely unlike either parent.

Similar to the implied terror of non-mutant parents in the X-men series, a world where xenogenesis can happen means that people may have children that are completely unlike them, and, therefore, completely unpredictable and perhaps even dangerous. Aside from the fear of what the children might be capable of (and the added troubles of the PCs perhaps having to do violence against children) you have the added level of knowing that your genes have not been passed on...your line has ended...you have no posterity.

My treatment of Halflings and Gnomes touch on this idea slightly. 

The former are children frozen in time, immortal and unable to age to maturity. Not only does this mean the end of a genetic line (no grandchildren for the halfling's parents), but it also means that you have hundred-year-old toddlers running around who have drastically outlived their parents and may be subject to all manner of social maladjustment that might be associated with immortality (watching you parents die without ever reaching the mental maturity to really process it). Ala Lord of the Flies, the PCs end up having to contend with violent tribes of children (likely young, adorable children).

The latter, Gnomes, freak everyone out because they hijack the reproduction of other creatures. Gnomes are more a disease than a race, unable to reproduce on their own, they infect others with their children. With just a glance, they can curse a woman of any race to only bear gnome offspring, without any sexual contact. Just the thought of this kind of violation can get under the skin of PCs...even if they never get around to reproducing, knowing that that gnome they ran into 5 levels ago has hijacked their future reproductive capability can get under their skin.

There has been at least one report of a human woman having a human, a gnome, and a halfling as children.
The free Lamentation of the Flame Princess adventure The Doom Cave of the Crystal Headed Children touches on this idea. It features cloned, hive-minded, mutant children that indirectly borrow genetic material from every woman in a town, and then alter the minds of those women to think of the children as their own. On its own this makes for an interesting gimmick for a one-shot adventure, but if you make sure to have a female PC be affected by it...then things get all kinds of interesting.

But of course you are asking, how can my PC do this to someone else (without being a gnome that is)?
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Yarash's Curse of Xenogensis I

(Alteration, Enchantment)
Level: 3rd
Range: 10 yds
Components: V, S
Duration: permanent
Casting Time: 1
Area of Effect: 1 creature
Saving Throw: Negates.

This spell permanently and erratically alters a victim's genetic material. Any time the victim sires or gives birth to a child, the offspring will be a new creature, completely unlike the parent. When the child is born, roll for a random creature using the table for the Monster Summoning I spell (found in the Appendixes of the Monstrous Manual).

The victim is allowed a save vs. spells to negate the effect. If the save fails, the curse is permanent until removed by a remove curse or wish spell cast by a caster of higher level the the wizard who cursed the victim.

This spell has no immediately visible or noticeable effects (though a proper villain should taunt the victim about the end of his line and so forth). Because of the nature of this curse, it may take years or even decades to manifest, if at all, but the victim will feel an immediate sense of impending dread any time they engage in procreative activities.  

Higher level version of this spell exist, causing more and more gruesome offspring. Curse of Xenogensis II-VII are identical to the spell above, but cause any offspring to become creatures from the corresponding monster summoning table.

So yeah, after years of trying, your PC and their mate have just given birth to a purple worm...roll initiative.