Ghouls are humans who have been exposed to FEV - but because they are already irradiated, it does not turn them into Super-Mutants, it turns them into zombie-like freaks... that's why the Super-Mutants were so concerned with finding "prime normals" (humans who have not been exposed to radiation) because the radition corrupts the effects of the FEV, turning a potential Super-Mutant, into a ghoul.
All ghouls & super-mutants are sterile. The males don't produce sperm, the females don't produce ova...they can't have children.