Wall Street Journal, Same-Sex Couples And The Marriage Penalty, by Laura Saunders:

Thanks to a 1996 federal law aimed at preserving traditional marriage, thousands of same-sex couples in California, Nevada, and Washington state could get big tax bonuses on their federal returns starting this year. The bonuses are off-limits to heterosexual married couples—a sharp reminder of the “marriage penalty” that often dings two-earner couples. …

All three states recognize domestic partnerships and also have what is known as community-property law. Community property refers to a system of ownership in nine states that usually attributes income and property acquired during marriage equally to both partners, regardless of who earned it. (The nine states are Arizona, California, Idaho, Louisiana, New Mexico, Nevada, Texas, Washington and Wisconsin.)

The three states also now apply community-property laws to registered domestic partners. So the IRS—which must follow state property laws—has ruled that these couples should figure their total community income and split it down the middle, starting in 2010.

That is where the benefit comes in. Although domestic partners must divide their income equally, the federal Defense of Marriage Act prevents the IRS from treating these couples as married joint filers. So for 2010 and after, each partner will claim half the community income but still file as single or head of household.

The result, in many cases, is a federal tax savings because a couple will avoid the marriage penalty that often raises taxes for two-earner heterosexual married couples.