Polygamy:
The problem with looking for proof texts for monogamy is that the Jews of Jesus time understood so clearly that it was wrong that Jesus devoted no time to teaching on it.

1. You are right in saying thinking of God's instructions in the Garden: "It is not good for man to be alone, so he created for Adam _a_ suitable helpmeet." We are told that a man should leave his mother and father and cleave to his _wife._

2. The polygamy of the patriarchs was _permitted_ but never condoned by God in the Old Testament.

3. The New Testament teaches than any church leader must be "the husband of but one wife." The New Testament also repeatedly teaches about a man's duty to his _wife_. There is never a positive mention of polygamy, certainly no commandment for it.

4. Experience and common sense proves that a man is hardly able to properly love and care for _one_ wife, let alone several.

5. Polygamy always arises in highly patriarchal systems like Mormonism and Isalm where women are treated as second class citizens.

6. Joseph Smith got the revelation on polygamy _after_ he was already heavily involved with many adulterous affairs.

7. Would _you_ like your husband to be a polygamist. :)