Esau doesn't stop there, though. Genesis 28:9 notes that Esau married Mahalath, who was the daughter of Ishmael. Since that chapter notes that Esau did this after realizing that his parents were displeased with the thought that Jacob might also choose his own wife from among the surrounding Canaanites, we can guess that he was somehow trying to win their favor by this choice.
Genesis 36:2 also adds two more names of women which Esau is said to have married: Adah and Oholibamah, who were Hittite and Hivite women. Some of the women named may be alternative names or nicknames for the same people. However, it seems that Esau had to have married at least four women, for Hittites (plural) account for at least two of the women, Mahalath was from Ishmael, and one woman was said to be a Hivite. That is at least four, and, if Esau can marry four women, then five is not such a stretch!
[I just ran into another problem. Basemath is said to be a daughter of Elon the Hittite in Genesis 26:34. However, in Genesis 36, she is said to be a daughter of Ishmael. Any thoughts? Are there two Basemaths?]
The New Testament book of Hebrews (12:16 NIV) warns, "See that no one is sexually immoral, or is godless like Esau, who for a single meal sold his inheritance rights as the oldest son." I imagine that he might be considered sexually immoral because he had so many wives, although, in a society where it seemed acceptable for a man to have several wives, I don't know why he would be especially singled out for this. Perhaps it is because the book of Hebrews is written from the godly perspective of 'one wife for one man', which is the original design for marriage. Genesis 2:24 (NIV) says: "...a man will leave his father and mother and be united to his wife, and they will become one flesh." If a man had five wives, what happens to the 'one flesh' aspect? Alternatively, Esau could have been promiscuous in other ways, I suppose, with women who were not his wives at all. At any rate, these verses about Esau and his wives contrast starkly with the godly way that his father Isaac had handled the events of the earlier part of the 26th chapter of Genesis.