It's not a question of state laws at all. We're talking IRS regulations. If you are not "legally married"--and the Feds only recognize legal marriages (common law or otherwise) between a man and a woman--taxes apply.
My sister is my registered companion since my wife died several years before I started work at AA. I have to pay the "imputed income" tax on the "fair market value" of every non-rev trip she makes. If you will go to Jetnet, Travel, Trip Book, Travel Service Charges, Imputed Income, you will find not only the rule, but the formula for calculating the imputed income tax. The rule is...
"Non-revenue travel for spouses, dependent children, and parents are not considered taxable income for you. However, travel by
Domestic Partner (DP), extended family, and friends generally is taxable and therefore generates imputed income when the service charges are less than the fair market value of the trip. Service charges and fair market values ("trip values") are shown in Sabre "WR" pricing entry and in the Jetnet Travel Planner's "NRSA Charges" section.
And, only D1/D2 travel by Domestic Partner or Registered Companion are considered "not fair market value." All D3 charges are considered fair market value.
Note: Domestic Partner, see also Gay Couple, or Straight couple "living in sin."