No. This is one situation where the marriage is not retroactive to the whole year. Medical insurance premiums that are paid by your employer to cover someone who is not your legal spouse or your tax dependent (such as a domestic partner) are considered taxable income to you, even if you never see the actual money because it is sent directly to the insurance company. You can stop the imputed income situation if you inform your employer of the legal marriage within 60 30 days of the marriage. But the money that was previously counted as taxable income because the person was not your legal spouse at the time the insurance premiums were paid (imputed income) is still counted as taxable income to you.
[Edited to correct: You have to notify the employer benefit plan within 30 days of a "change of status event" to change your plan. Otherwise, you have to wait for the next open enrollment. US Treas. Reg. 1.125-4.]
