Critics worry the legislation undermines hard-won rights for women enacted after the fall of the Taliban's strict Islamist regime.
The law -- which some lawmakers say was never debated in parliament -- is intended to regulate family life inside Afghanistan's Shiite community, which makes up about 20 percent of this country of 30 million people. The law does not affect Afghan Sunnis.
One of the most controversial articles stipulates the wife "is bound to preen for her husband as and when he desires."
"As long as the husband is not traveling, he has the right to have sexual intercourse with his wife every fourth night," Article 132 of the law says. "Unless the wife is ill or has any kind of illness that intercourse could aggravate, the wife is bound to give a positive response to the sexual desires of her husband."
One provision also appears to protect the woman's right to sex inside marriage saying the "man should not avoid having sexual relations with his wife longer than once every four months."
The law's critics say Karzai signed the legislation in the past month only for political gains several months before the country's presidential election.
The United Nations Development Fund for Women, or UNIFEM, said the law "legalizes the rape of a wife by her husband." "The law violates women's rights and human rights in numerous ways," a UNIFEM statement said.
The U.S. is "very concerned" about the law, said State Department spokesman Robert Wood. "We urge President Karzai to review the law's legal status to correct provisions of the law that limit or restrict women's rights."
Wood added that U.S. Secretary of State Hillary Rodham Clinton had met with female Afghan lawmakers in The Hague and had assured them that "women's rights are going to be paramount in this administration's foreign policy, not an afterthought."
Canada's Defense Minister Peter MacKay said he will use this week's NATO summit to put "direct" pressure on his Afghan counterparts to abandon the legislation.
The issue of women's rights is a continuous source of tension between the country's conservative establishment and more liberal members of society. The Taliban government that ruled Afghanistan from 1996 to 2001 banned women from appearing in public without a body-covering burqa and a male escort from her family.
Much has improved since then. Millions of girls now attend school and many women own businesses. Of 351 parliamentarians, 89 are women.
But in this staunchly conservative country, critics fear those gains could easily be reversed.
Fawzia Kufi, a lawmaker who opposed the legislation, said several of its articles undermine constitutional and human rights of women as equals and take the country backward.
"All the efforts that were made in the last seven years to enhance women's rights will be undermined," Kufi said.
Karzai has not commented on the law. A spokesman, Waheed Omar, said the president is "aware of the discussion surrounding the law, and is looking into the matter."
Brad Adams, the Asia director for the New York-based Human Rights Watch, said the law is a "dramatic setback for women's rights."
"It directly contradicts the freedoms enshrined in the Afghan constitution and the international conventions that Afghanistan has signed up to that guarantee the rights of women," Adams said.
Safia Sidiqi, a lawmaker from Nangarhar province who condemned the legislation, said she cannot remember parliament debating or even voting on the law and she does not know how it came to be signed by Karzai. She called for the law to be recalled to parliament for debate.
"It is impossible in a two-month session for parliament to pass a law more than 200 pages long," she said of the 263-page law.
Sayed Hossain Alemi Balkhi, a Shiite lawmaker involved in drafting it, defended the legislation saying it gives more rights to women than even Britain or the United States does. He said the law makes women safer and ensures the husband is obliged to provide for her.
As Karzai seeks re-election later this year, he is courting voters in the Shiite community, Kufi said. Women voters are presumed to vote as their husbands do.
"Women's basic freedoms are being sacrificed for the political and electoral gain of a few parliamentarians," Human Rights Watch's Adams said.
Associated Press writers Rob Gillies and Charmaine Noronha in Toronto and Matthew Lee in Washington contributed to this report.