First Off... I'm Mormon.
The Church of Jesus Christ of Later-Day Saints (LDS for Short/Mormon is common in media) no longer supports the practice of Polygamy. That was over in like 1870ish. (I am not at home and don't have the date in front of me.) It has been a long time since The LDS Church has supported Polygamy.
Also, as the name of our church suggests we believe that Christ is the son of God and was sent here to suffer, bleed, and die for us. Correct me if I am wrong, but isn't that the very basic definition of Christianity?
Lastly, If any of you have serious, legitimate questions for me, I can answer them on this thread, via PM, or even Email if you so choose. I will do my best to answer them, but since I am not perfect, and my knowledge is not perfect my answers may not be what you are looking for. But I am more than willing to point you in the right Direction for answers.
EDIT: To answer the OP's question, not that I know of, not within CK2's timeline at least. I know for a gameplay reason that I would love to see fictional Heresies that can spring up if the right circumstances are met.