D&C 132

The Lord reveals to the Prophet Joseph Smith -

"...I am the Lord thy God; and I give unto you this commandment—that no man shall come unto the Father but by me or by my word, which is my law, saith the Lord." (Doctrine and Covenants Section 132:12)

In other words we must be obedient to the words of Christ to live with the Father. The Lord explains that -

"Abraham received all things, whatsoever he received, by revelation and commandment, by my word, saith the Lord..." (Doctrine and Covenants Section 132:29)

Abraham was obedient when -

God commanded Abraham, and Sarah gave Hagar to Abraham to wife. And why did she do it? Because this was the law; and from Hagar sprang many people. This, therefore, was fulfilling, among other things, the promises." (Doctrine and Covenants Section 132:34)

The Lord even asks -

"Was Abraham, therefore, under condemnation? Verily I say unto you, Nay; for I, the Lord, commanded it." (Doctrine and Covenants Section 132:35)

Also -

Abraham was commanded to offer his son Isaac; nevertheless, it was written: Thou shalt not kill. Abraham, however, did not refuse, and it was accounted unto him for righteousness." (Doctrine and Covenants Section 132:36)

Because he was obedient to the Word. According to the Lord's command -

Abraham received concubines, and they bore him children; and it was accounted unto him for righteousness, because they were given unto him, and he abode in my law; as Isaac also and Jacob did none other things than that which they were commanded; and because they did none other things than that which they were commanded, they have entered into their exaltation, according to the promises, and sit upon thrones, and are not angels but are gods.
38 David also received many wives and concubines, and also Solomon and Moses my servants, as also many others of my servants, from the beginning of creation until this time; and in nothing did they sin save in those things which they received not of me." (Doctrine and Covenants Section 132:37 - 38)

Jacob explains concerning plural marriage -

"...my brethren, hear me, and hearken to the word of the Lord: 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, delight in the chastity of women. And whoredoms are an abomination before me; thus saith the Lord of Hosts.
29 Wherefore, this people shall keep my commandments, saith the Lord of Hosts, or cursed be the land for their sakes.
30 For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things." (Book of Mormon Jacob 2:27 - 30)

Again it comes down to following the word or commands of the Lord. Some have a difficult time when Nephi was commanded to slay Laban, but it too was considered righteousness because the Lord commanded it. The Lord has clearly stated -

"Go ye, therefore, and do the works of Abraham; enter ye into my law and ye shall be saved." (Doctrine and Covenants Section 132:32)

"For I am the Lord thy God, and ye shall obey my voice..." (Doctrine and Covenants Section 132:53)