Adding on to Lost One's post, remember that Lúthien's mother was an Ainu, and evidently Melian was "properly equipped". [img]smilies/rolleyes.gif[/img]
Tolkien makes another point about Morgoth, related to the earlier conception in which Arien was the object of his lust (the observation probably applies to the later conception of the Lúthien/Morgoth scene as well).
Quote:
Evil is fissiparous. But itself barren. Melkor could not 'beget', or have any spouse (though he attempted to ravish Arien, this was to destroy and 'distain' her, not to beget fiery offspring).
Morgoth's Ring--Myths Transformed
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So Morgoth could not have had children with Lúthien--not because he was a Vala, but because was evil to the point of being unable to perform any creative or procreative act.