A porn virus is a concept made up by males that means the reason there is porn all over one's computer is because they got a virus.

100% of the time it is used as a scapegoat since the real reason there is porn on one's computer is due to the fact that the user voluntarily looked it up.

Thankfully, such viruses do exist, and have therefore saved countless asses of boys and men alike from pissed off parents, girlfriends, or wives who find porn on their computers.

Pissed off mother: Johnny, why is there porn on your computer?!
Johnny: It was a porn virus, I swear!

Pissed off Girlfriend: You've been looking up porn! Am I not good enough for you or something?!
Boyfriend: OH NO, I've been targeted with a porn virus like all my friends!