According to the New York Times:

 

"Supporters of gay rights have long been trying to change the tax treatment of health benefits provided by employers to the domestic partners of their employees. In effect, such benefits are now treated as taxable income for the employee, and the employer may owe payroll taxes on their fair-market value.
Under the bill, such benefits would be tax-free, just like health benefits provided to the family of an employee married to a person of the opposite sex."

Looks to me like a bit of liberal incrementalism at work here.