Jack and Marcie, “Friday the 13th.”
A young Kevin Bacon seeks refuge in a bunkhouse with his lady. They commence to humping, but sadly both are murdered by a shadowy figure before they can re-connect for more sex. Horny teenagers are like rabbits, so it’s a safe bet that the pair would have hooked up again before the end of the camping trip. But alas, an arrow through Kevin’s throat forever cockblocked him.
Gibb and Trey, “Freddy Vs. Jason.”
Two kids with, let’s be honest, dumb names. Maybe limiting them to one sexual encounter was enough. Jason shows up to stab the hell out of Trey, who’s basking in post-coital bliss, and then fold him up backwards in a collapsible bed. An evil cockblock, sure, but maybe a good one in the Darwinian sense.