Jacob's Concubines and Wives

...He was a boy with the sons of Bilhah and Zilpah, his father's wives... (Genesis 37:2)

Jacob's concubines, Bilhah and Zilpah, were concubines, but also fully his wives. The Hebrew word used here literally just means "woman," so translating it as some other word in English is a judgment call requiring the translator to make some assumptions about the original intent. Moses might have been just calling Bilhah and Zilpah Jacob's women, but it is the same word used in other passages to refer to Eve, Sarah, Hagar, Keturah, Rachel, Leah, and all of the other wives of the Bible, so the word "wives" is perfectly justified in this verse.

A concubine was not just a spare sex partner, but had rights and responsibilities in her husband's house. The primary difference between a concubine and any other wife in the Bible is that a concubine is also a slave. Keeping in mind that Biblically a slave has more rights than free men in most other ancient cultures, it certainly isn't an ideal relationship, but it's not the horrifying lot anti-patriarchs make it out to be either.