Source:JSP:Although he hated adultery and was deeply loyal to his wife Emma, he believed he was to take additional wives as had the ancient patriarchs
Joseph Smith Papers: "Although he hated adultery and was deeply loyal to his wife Emma, he believed he was to take additional wives as had the ancient patriarchs"
Joseph Smith Papers Project (online):
At times revelation became a burden as well as a blessing, at no time more than when plural marriage was revealed. Plural marriage was the final component of the logic of restoration. Smith had prayed for an understanding of Old Testament polygamy and was commanded to do the “works of Abraham.”45 Although he hated adultery and was deeply loyal to his wife Emma, he believed he was to take additional wives as had the ancient patriarchs. He went about it carefully, one woman at a time, usually approaching her relatives first and going through a prescribed wedding ceremony. During his lifetime, he was married to approximately thirty women.46 Although conjugal relations were apparently involved, he spent little time with these women, the need for secrecy and the demands on his time keeping them apart. At first aghast at what her husband was doing, Emma eventually agreed to a few of the plural marriages but then pulled back. She oscillated between hesitant submission and outright opposition to the practice, but according to Maria Jane Johnston Woodward, who worked for a time as a servant in the Smith household, Emma told her, “The principle of plural marriage is right. . . . [I]t is from our Father in Heaven.” After her husband’s death, Emma refused to go west, where plural marriage would be practiced. She never admitted to her children that their father had been involved. 
- "Joseph Smith and his Papers: An Introduction," josephsmithpapers.org (accessed 24 April 2012): p. 6 of 9.