Referring to the Wikipedia article I think you are yourself referencing:
According to the Malleus Maleficarum, or "Witches' Hammer", written by Heinrich Kramer (Institoris) in 1486, succubi collect semen from men they seduce. Incubi, or male demons, then use the semen to impregnate human females,[13] thus explaining how demons could apparently sire children despite the traditional belief that they were incapable of reproduction. Children so begotten—cambions—were supposed to be those that were born deformed, or more susceptible to supernatural influences.[14] While the book does not address why a human female impregnated with the semen of a human male would not produce regular human offspring, an explanation could be that the semen is altered before being transferred to the female host. However in some lore, the child is born deformed because the conception was unnatural.
This is more likely the reason -- Mr. Bad Touch and artificial insemination are naughty AND unsanitary. These demons are Pro-Lifers.
Thrall to the Wire of Self-Excited Circuit.
