You are in a situation called "imputed income." If you get married, your spouse is entitled to certain tax free employee benefits. Or, if your DP can be your tax dependent, their benefits can be tax free. But because your DP is not your spouse and not your dependent, your employer can't provide tax-free benefits for them.
Let's say as an example, you have single health insurance that costs you $100 per month and your employer pays an additional $400 per month. Your premium is pre-tax, and your employer's share is deductible on their corporate tax return. To add a DP who is not entitled to tax-free benefits, the employer charges you an extra $100 for your share of their premium, but this is an after-tax deduction. The employer pays their $400 share, but because it can't be tax-free, it is considered taxable income to you, as if you had gotten a $400 per month raise and used it to pay the insurance premiums. The employer will withhold extra income tax and extra social security tax from your paycheck as if you had received a $400 raise.
As a result, your take home pay will go down, possibly by quite a lot (up to 40% of the value of the benefit, depending on your tax bracket and the state you live in.) Your W-2 at the end of the year will show an increased salary, again because the tax effect of DP benefits is the same as if you got a raise and paid for the benefits yourself. Your W-2 will show the value of benefits as additional box 1 income, "imputed income."
The extra withholding that will be taken out of your check should cover the tax you owe so you should not see a large tax bill next year, but you will see a decrease in your take home pay.
This does not apply if the DP can be your tax dependent (lives with you all year, and earns less than $4100 for 2017). And the issue of imputed income stops if you get married, as soon as your tell your employer, so they can switch the person over to spousal tax-free plan.
