The benefit is to reduce the tax burden on married gay couples. There is no burden for single people or straight married couples.
Single person == Contributions to health insurance are not taxed.
Single Person w/Children == Contributions to health insurance are not taxed.
Married Straight Couple == Contributions to health insurance are not taxed.
Married/Cohabitating Gay Couple == Contributions to health insurance are taxed.
Unmarried straight couple == Contributions to health insurance not taxed.
Unmarried straight couples can... get married to avoid this tax. Gay couples can not.
Before my wife and I were married she was on my insurance as a domestic partner and I did pay the tax.
When we got married, I no longer had to pay the tax.