I think part of the problem here is that Tolkien, bless his heart, keeps forgetting that you need women as well as men to make descendants. He can't help it!
Remember his original list of seven (male) Dwarves - "the fathers of the fathers" put to sleep? Later, of course, he must have had this pointed out to him and added a mate for each (except Durin, who then inexplicably became an ancestor too). All those "fathers of the fathers of ..." without any mothers. So, somehow, that old bachelor were-bear Beorn, who seems to need only the company of his pets, became the ancestor of a sturdy "race" of Men. That's Tolkien for you.
I just suspend disbelief and remember Tolkien and his boys' club at Oxford. (g)