The measure, aimed at creating different laws for Iraq's majority Shiite population, could further fray the country's divisions amid some of the worst bloodshed since the sectarian fighting that nearly ripped the country apart after the US-led invasion.
It also comes as more and more children under 18 get married in the country.
"That law represents a crime against humanity and childhood," prominent Iraqi human rights activist Hana Adwar said. "Married underage girls are subjected to physical and psychological suffering."
Iraqi law sets the legal age for marriage at 18 without parental approval. Girls as young as 15 can be married only with a guardian's approval.
The proposed new measure does not set a minimum age for marriage.