Today I was reading a little HoME (Morgoth's Ring Part Three, Second Phase in the section entitled Laws and Customs Among the Eldar) and I spotted something of relevance-
Quote:
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there is no record of any among the Elves that took another's spouse by force... one so forced would have rejected bodily life and passed to Mandos.
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Since the situations are comparable, it leads me to believe that if Morgoth or any of his minions attempted to force themselves on Luthien her spirit would leave her body (meaning that Morgoth and friends would have to settle for necrophilia).