The books are a single set of stories within the Middle Earth realm. Although it would be plausible to say something seen in the books definitely happened, it's wrong to say something not in the books definitely did not happen. When addressing things that did not happen in the books, you have to rely on common sense more than lore, since the fact that it is not in the books means it's in a blind spot of the lore, rather than an impossibility. Such blind spots are completely open to interpretation from whoever notices them. Now, although the books don't address intermarriage between elves and humans outside of the one, since it did happen in the books at least once, it is plausible to say that such intermarriages did happen, though perhaps not frequently.