He is most certainly a man, a fallible man who even receives chastisement from God in scriptures that he himself dictated.

Polygamy is an interesting issue, as it was practiced in OT times, but by the NT it had died out. I'm sure there are numerous articles I could link to, but for now I will start with the best answer that comes from our Book of Mormon, found in Jacob 2, with my emphasis placed on the last verse:

23 But the word of God burdens me because of your grosser crimes. For behold, thus saith the Lord: This people begin to wax in iniquity; they understand not the scriptures, for they seek to excuse themselves in committing whoredoms, because of the things which were written concerning David, and Solomon his son.

24 Behold, David and Solomon truly had many wives and concubines, which thing was abominable before me, saith the Lord.

25 Wherefore, thus saith the Lord, I have led this people forth out of the land of Jerusalem, by the power of mine arm, that I might raise up unto me a righteous branch from the fruit of the loins of Joseph.

26 Wherefore, I the Lord God will not suffer that this people shall do like unto them of old.

27 Wherefore, 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.

Long story short, it's almost always wrong, with the exception largely to raise a population of God's people.