Polygamy

David said:

We denounce the doctrine of polygamy and spiritual wifeism. It is a great evil, shocking to the moral sense, and the more so because practiced in the name of Religion. It is of man and not of God, and is especially forbidden in the Book of Mormon itself in these words:

 
24 Behold, David and Solomon truly had MANY WIVES and CONCUBINES, which thing was ABOMINABLE BEFORE ME, saith the Lord.
27 For there shall not any man among you have save it be ONE WIFE; and CONCUBINES he shall have NONE;
28 For I, the Lord God, delighteth in the chastity of women... (Jacob 2)

In the New Testament, a church leader could NOT be a polygamist:

2 A BISHOP then must be blameless, the HUSBAND OF ONE WIFE, vigilant, sober, of good behaviour, given to hospitality, apt to teach;
12 Let the DEACONS be the HUSBANDS OF ONE WIFE, ruling their children and their own houses well. (1 Timothy 3)
5 For this cause left I thee in Crete, that thou shouldest set in order the things that are wanting, and ordain ELDERS in every city, as I had appointed thee:
6 If any be blameless, the HUSBAND OF ONE WIFE, having faithful children not accused of riot or unruly. (Titus 1)

Joseph disobeyed the Book of Mormon and the New Testament when he allowed church leaders to practice polygamy and made it a requirement of salvation.

Did Emma know about Joseph's polygamous affairs? Yes!

52 And let mine handmaid, Emma Smith, receive ALL THOSE THAT HAVE BEEN GIVEN (past tense) unto my servant Joseph (D&C 132)

Further Reading

Finally, the Mormon Church has admitted that Joe's marriages were both sexual in nature and were to already married women (polyandry):

For a Church-approved work to state that polygamy involved “conjugal relations” and use the term polyandry is landmark. (The Joseph Smith Papers, Journals, Volume 2: December 1841--April 1843)