Guy who’s infected with a virus that slowly turns him into a monster
The very first symptom before anything else is a greatly increased libido. At first it doesn’t seem serious, and he just jerks off a lot more. But the libido keeps increasing, and he starts to have a hard time keeping up with it. It gets bad enough to be concerning, but it’s hard to think about anything other than relieving himself.
Then the physical changes start. First it seems like his cock is bigger than it was before, and he’s cumming a lot more than any human should. The rest of his body follows soon after. Growing larger and stronger, warping into a scarier form. Maybe it’s a layer of fur and a tail, tough scales, or slimy tentacles. He can’t focus on anything else but the throbbing of his cock, and now he can’t even satisfy himself from jerking off. The only thought on his mind is to find a mate. To breed. To fill them with his young.
And so he goes out at night, no longer recognizable as a human. The first fertile human he sees is easily caught and dragged into a secluded location, where he rips their clothes off and rams his cock inside. Madly thrusting away, already addicted to the feeling of their insides squeezing around him. Maybe the human struggles, tries to kick and punch and fight him off, but he doesn’t even notice. When he cums, he fills them with more than their body should handle. Stuffing their belly full until they already look pregnant. Satisfied with his work, he takes the human back to his home where he’ll continue to breed them some more. Later on, their belly starts to swell with his equally inhuman young. Once they give birth, he’ll be right there to pump them full of babies again.
Alternate idea: The guy actually has a partner, who gets concerned that he hasn’t come out of his room in awhile. But once they head inside, they see this huge horny monster that used to be their partner. And he sees an eligible mate that can carry his offspring. So they get to be the lucky captive, wrecked by his giant cock and filled with his thrashing kids.