Clarified
they are not Christian
They are not -
"This is not just another Church. This is not just one of a family of Christian churches. This is the Church and kingdom of God, the only true Church upon the face of the earth..."
- Prophet Ezra Taft Benson, Teachings of Ezra Taft Benson, p. 164-165
that they belong to a cult
Yes, they deny every essential doctrine of Christianity and meet social criteria for a cult as well as religious criteria.
that they are insular and secretive,
Are all your temples now open to the non-recommend public as well as your 'secret/sacred' ceremonies including the little creation film/play?
that they practice polygamy
Hmmmmm D&C Section 132 is still mormon scripture and orthodox mormons practice it.
“This is the Church and kingdom of God, the only true Church upon the face of the earth...”
All the other Churches believe that about themselves. Why shouldn’t Mormons?
Yup. Mormons can't dump D&C 132 cause it's their ONLY "scripture" teaching that marriage is eternal; and one of few "scriptures" they have which teaches the concept of plural gods.
They are stuck with it.
Which means the fLDS are in reality the "orthodox" Mormons in practicing polygamy; whereas the LDS are the breakaway Mormon cult-sect.