I practice in Virginia, and until recently was of the opinion that cohabitation violated local law and therefore prohibited a person from claiming a person of the opposite sex as a dependent. However, when this question was asked of the Virginia Department of Taxation recently, this is what they posted in their FAQ's site:
"6. In Virginia, does cohabitation violate local law? Specifically, can one person claim another as a dependent if that person has no income and lived with the taxpayer all year?
Neither of these issues is a matter of Virginia tax law. If a filer properly claims another person as a dependent for federal purposes, that person may also be claimed as a dependent on the Virginia return. However, the determination as to whether such a claim is proper on the federal return is a matter of federal tax law."
If you care to look at the whole site, it is http://www.tax.virginia.gov/site.cfm?alias=TaxProfessionals and go to Tax Professionals FAQ. After this response, I am not sure what to tell people. But if it is a matter of federal law, and if federal law states except where in violation of local law, it seems to me that you still can't claim your significant other in VA.