Nature has some pretty strange sexual practices, but a species of Australian sea slugs takes the cake. When this lovely creature inseminates its mate, he also stabs it in the head and injects mind-altering hormones.
This sea slug, Siphopteron species 1, has completely strange biology as well as sexual habits. They are simultaneous hermaphrodites, meaning they possess both male and female reproductive systems and use both during sex. So in the end, both partners get stabbed and pregnant. Sounds like quite the arrangement...
As strange as that is, it gets weirder: They have two penises. One has six spikes and is responsible for the transfer of sperm into the female genital opening. The other penis is used to stab the mate in the head and inject prostate fluids.
I'm not sure which one is more terrifying.
The injection of prostate fluid occurs very close to the central nervous system and seems to have effects on the behavior of the animal. The authors of this study have plans to test this more directly, but for now, it looks like the sea slugs who are injected with prostate fluid no longer act as females, which ensures that they are not impregnated by another sea slug.
While this puncture wound is less romantic than cupid's arrow, it's certainly the most direct way to control paternity.
Hats off to your problem solving skills, evolution.