It's unlikely flies would lay their eggs on a healthy/living animal to begin with. Many species of maggots will only eat dead tissue (thus their already-mentioned use in wound care), so if the eggs did manage to hatch on intact skin, they'd find little to eat.
OTOH, if the hypothetical girl had really bad acne, for example, you could chain together enough circumstances where it would almost be plausible, though highly improbable, that she passed out covered in semen, some random fly laid eggs in the semen, blah blah. The huge HUGE red flag is the "could only happen if he'd had sex with a corpse" bit. Spontaneous generation was given up a couple centuries ago. Maggots don't come from rotting meat, they come from fly eggs.
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Simple Machines in Higher Dimensions
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