The reason for why I did so is that *Tyrannosaurus* is a genus name and *rex* a specific epithet. On the species level, genus names _are_ proper names, even though they aren't on their own level. ~:-)
Any taxon name represents a large number of individuals, as such they are not Eigennamen but nouns for a class of items with some shared characteristics, but unlike 'house', which denotes a single such item out of all 'houses'. The White House is an Eigenname, although more White Houses exist and of course many white houses. Tyrannosaurus is not this kind of name. Thus it is improper to speak of Tyrannosauruses. Tyrannosaurus is the sum of all 'Tyrannosauruses', the plural is inherent in the nominative singular word. And again, on the species level: Tyrannosaurus rex is not The King.
Othnielia rex formerly was Nanosaurus rex (male).
Edmarka is named after William Edmark (male) [and Othnielia after Charles Othniel Marsh], so, if one follows your logic, "Edmarka rex" should have been named Edmarkus rex in the first place.
Anyhow, the gender issue is a difficulty which is not felt in every culture the same.