That definition of "normal" is irrelevant to the biological effects of mutation on fertility.

Here: If the human reproductive system has evolved over millions of years in a condition where the vast majority of the population are one of either two sexes, with particular chromosomes even being present in only one of the sexes, then a mutant that somehow ends up with both sexual organs is almost certainly going to having all sorts of problems fertility-wise. That kind of mutation breaks the assumptions the reproductive systems have evolved under (assumptions such as amounts of testosterone/oestrogen/whatever other hormones in the blood, the physical arrangement of the sex organs, and probably all kinds of other stuff).