Already answered Not a question Bad question Other. Tips Mormon girls are much like other girls their age, in that they want guys to respect them and treat them well.
She will not marry you until you convert to mormonism whole cloth.
I don't want to advocate manipulation but asking questions concerning Joseph's dishonesty about polygamy to his first wife and his marriage of other men's wives and marriage of a 14 year old may prove more fruitful than just saying wow polygamy is messed up.
It will definitely take patience to work through any of this with her but it sounds like she's a pretty awesome person.