It's called imputed income.
Your employer can provide tax-free health insurance to you, your spouse, and your dependents. Tax-free in this case means that you don't pay income tax on the employer share of the premiums and your share of the premiums is deducted from your check pre-tax.
When employers offer domestic partner benefits and the partner is not a dependent, the health insurance for the partner can't be tax-free. That means that the employer share of the premium is taxed as if the employer gave you a raise in that amount and used the raise to pay the premiums. And your share of the partner's premium is deducted after-tax, not before tax. The employer's share will be included in your W-2 box 1 as taxable wages and you will pay social security, medicare, and state and federal income tax. You should be seeing higher withholding from your check to cover these taxes, so it shouldn't make too much impact on your refund at the end of the year, but you will see less take home pay due to the withholding.
You can remove the imputed income and the tax consequences if you marry your partner or if your partner qualifies as your dependent. To be a dependent, the partner must live in your home the entire tax year, you provide more than half their living expenses, and their taxable income is less than $4050. (You don't get a refund of the tax, but you can stop the imputed income going forward, by getting married or notifying the employer that your partner qualifies as a dependent.)
