Well, if one is not willing to entertain the possibility of infidelity in Tolkien's subcreation (now, why did I know that this would be contested?
), there is the possibility of some other, unnamed (and previously unattached) Maia, of Ulmo's service, as a parent or grandparent of Goldberry's, rather than Uinen. I would consider this more likely, actually, all the way around, since we do know that the Maiar were vastly more numerous than the Valar, and more "available," so to speak, since their roles (and lives) within Ea are not as well-known as those of the Valar. Although I don't recall that Tolkien explicitly said that the Ainur do not reproduce among themselves, I believe this is strongly implied by the fact that early versions of his mythology have the Maiar as the children of the Valar, and in the later, more finished versions, this concept disappears completely. Only Luthien, the child of a Maia and an immortal incarnate, remains. This is given such prominence in Elvish lore, it is easy to believe that this is the ONLY such incident, and thus Luthien is the only child ever literally born of the Ainur. But given that Tolkien writes his books as if they are the histories and literature compiled by the inhabitants of his created world, there is always the possibility of stories unwritten, tales untold -- not to mention bias on the part of the "authors," who might tend to favor certain tales over others. Neither Tom nor Goldberry have the (apparent) historical prominence of Melian, Elwe, Luthien, or Beren, and thus there may be much of their story that is either unknown, or considered insufficiently interesting to publish. Although Tom is known to the Elves, he seems a greater part of the lore of the Hobbits (not surprising, given where he lives), and the Hobbits, being as unfond of "adventure" as they are, would likely know precious little about Tom's past, as well as his Lady's. The Elves (or at least the Elves who wrote the books we have seen) might simply have not been interested in Tom, because he does not appear to have
done anything that might interest an Elvish writer. For a Hobbit, who enjoys more mundane song and dance and "nonsense," tales of Tom, real or imagined, might be much more appealing. (And this is not intended to denigrate Elvish writers, just an attempt to find an explanation for why so little is known, or was written, about Tom and Goldberry.)