Vance, I checked Dirr and yes the vast majority of Junipers are Dioecious, with some individuals being monoecious ( typically a small percentage). Where the problem lies is in your translation of dioecious, where you state dioecious means both sexes on same plant. Actually, monoecious means, one house, both sexes reside in same individual and dioecious means, two houses, where the sexes are separate and distinct. Quoting Dirr's definition of dioecious (5th Edition, Manual of Woody Landscape Plants, Glossary page 1132): "Dioecious: having unisexual fls.,each sex confined to a separate plant, said of species."
So those who state they have male or female Junipers are most likely stating the truth. For example, with California Junipers only about 2% are thought to be monoecious, haven't seen one yet in Bonsai.