The changing status of domestic partnerships also affects unmarried heterosexual couples, who face similar problems in obtaining health insurance and death benefits.

Most laws which affect couples are within provincial rather than federal jurisdiction.

"Today he's going to talk about taxes and the way that the marriage penalty affects married couples," she said.

The measure would not affect couples who enter into a separation agreement before filing for divorce.

The issue not only affects unmarried couples but also handicapped, elderly and other single people living in group homes.

It would affect less than a quarter of the nation's Social Security recipients - single retirees with incomes above $25,000 and couples with incomes over $32,000.

The penalty primarily affects couples in which the two partners have roughly equal incomes.

This affects couples filing jointly with taxable incomes of $29,750 to $71,900 and single taxpayers with $17,850 to $43,150.