I know the origin but in
France the names are
Augustin and
Constantin are used, you would never say "St.
Augustine" and even though I'm from the States I guess I've lived over there enough that the names are now male to me as well
Augustine and
Constantine are only used for girls and in fact I had this discussion with a French friend recently who actually finds the "e" ending offensive for the Saint and my dh thinks it's strange so it really does depend on where you're from!
However your explanation of the name and why the "E" was tacked on makes a lot of sense and I think you're right. Plus there are more male names that end with "ine" though I can't think of any. I would also add that other societies tend to use a masculine ending but English is different so it's OK to assign whatever gender you want to a name? Not sure but that's my only other guess.
Thanks for your info!