The way that I see it, Melian was the only Maia to fall in love with an Elf. She was probably given a special dispensation of some sort so that she could permanently inhabit her fana, as one of the Elves. She still retained her "special powers" as a Maia, but she assumed the body of an Elf (depending on which description you read).
As for Ungoliant, if we stick to the theory that Maiar cannot reproduce among themselves, then what about the fact that she was the Mother of All Spiders (figuratively and literally). It is said that she mated with the lesser spiders, and devoured them, and thus, produced a brood of spiders, like the Mirkwood spiders. We're not sure what those lesser spiders were, if they were escaped creations of Morgoth or something like that. But if they were also Maiar, or at least lesser Maiar inhabiting the forms of spiders like Ungoliant, then they couldn't have reproduced.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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