Pervert Magic

A twisted apocalypse born of the chaste's worst nightmares, interdimensional beings all too similar to the worst Christian propaganda have invaded the earth and flooded it with some form of biological nanoswarm running on ambient energy undetectable through any affordable and widespread technology. What's worse is it's tendency to react violently to sperm for some unknown reason.

In layman's terms, Demons invaded through portals and made male masturbation a form of suicide.

Humans fared surprisingly well on average during the initial waves. Most nations managed to rally defensive lines or at least buy time for an evacuation to a fortified area. A few million more deaths than usual across the several billion people on earth isn't actually all that bad when dealing with a planetary invasion no one was expecting.