Genesis 30:9

When Leah saw that she had left bearing, she took Zilpah her maid, and gave her Jacob to wife.

Leah gave her maid Zilpah to Jacob to be his wife out of jealousy toward Rachel. That was the same jealousy that had already moved Rachel to give her own maid to Jacob. The giving of the two maids tangled and confused the family relationship and only added to the unhappiness of both Leah and Rachel. That growing unhappiness proves polygamy to be wrong, and Laban himself knew the jealous rivalry that lay behind the gifts of the maids and the unhappy result they produced.