Although the Lord has been known to call men out of obscurity to be his leaders, once called I believe he expects them to repent and be clean. They may not overcome every small sin that man can do but I think he expects them, at a bear minimum, to not do all the major sins like murder, adultery, homosexuality, fornication, etc., etc. God expects his servants to repent of such things or lose their calling.
Isaiah 52:11
11 Depart ye, depart ye, go ye out from thence, touch no unclean thing; go ye out of the midst of her; be ye clean, that bear the vessels of the Lord.
Saul of Tarsus is an interesting example. He persecuted the Christians and while doing so, God called him:
Acts 22:6-16
6 And it came to pass, that, as I made my journey, and was come nigh unto Damascus about noon, suddenly there shone from heaven a great light round about me.
7 And I fell unto the ground, and heard a voice saying unto me, Saul, Saul, why persecutest thou me?
8 And I answered, Who art thou, Lord? And he said unto me, I am Jesus of Nazareth, whom thou persecutest.
9 And they that were with me saw indeed the light, and were afraid; but they heard not the voice of him that spake to me.
10 And I said, What shall I do, Lord? And the Lord said unto me, Arise, and go into Damascus; and there it shall be told thee of all things which are appointed for thee to do.
11 And when I could not see for the glory of that light, being led by the hand of them that were with me, I came into Damascus.
12 And one Ananias, a devout man according to the law, having a good report of all the Jews which dwelt there,
13 Came unto me, and stood, and said unto me, Brother Saul, receive thy sight. And the same hour I looked up upon him.
14 And he said, The God of our fathers hath chosen thee, that thou shouldest know his will, and see that Just One, and shouldest hear the voice of his mouth.
15 For thou shalt be his witness unto all men of what thou hast seen and heard.
16 And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord.
Now if Saul, later called Paul, had turned back to his ways of persecution, I doubt that the Lord would have continued with him in his wickedness. The same would apply the Abraham, Isaac and Jacob. Yet they continued in their ways of having more than one wife and in the eyes of God these men were justified. Abraham himself being known as the father of the faithful. Would Abraham have been a faithful servant if his acts were immoral and unjust in the eyes of God? I think not and he would not have received the great blessing that through him and his seed would all the world be blessed if he were not a faithful servant.
Does this mean that these men were perfect? No, but they lived the laws of God to the extent that God found them worthy to bless them as leaders in the cause of righteousness. The revelations and lives of these men are condensed into a few chapters of a single book in the bible. All that God spoke to them and taught them is not revealed in the Book of Genesis. Through the prophet Joseph Smith, God made it know what his law was regarding plural marriage and can be found in the
section 132 of the Doctrine and Covenants.