[The Saucepan Man]

As far as I am aware, there is nothing to suggest that Melian was the only incarnate Maia capable of reproduction. Equally, I am not aware that there is anything to suggest that one incarnate Maia could not mate with another incarnate Maia.

But it does beg the question as to why Balrogs did not breed amongst themselves. Perhaps the answer is that they were all took male form. Those who are referred to specifically certainly seems to be male.

But that does not preclude the possibility that Balrogs bred with Elves or Humans, or possibly even Orcs, to produce offspring which, while not as powerful as fully-fledged Balrogs, would nevertheless be pretty powerful in their own right.