From a pro-life website, so take it for what that's worth:
The main ingredient of Plan B is the steroid levonorgestrel. The abortion drug, RU-486, is also a steroid because its main ingredient is mifepristone. Because they come from the same chemical family, the molecular formulas for both drugs are very similar. The formula for Plan B is C21H28O2 while the formula for RU-486 is C29H35NO2.
Plan B can be taken up to three days after conception. The Plan B product information indicates that the main purpose of the drug is to prevent ovulation or fertilization, but the statement that fully reveals the danger of Plan B reads, “In addition, it may inhibit implantation (by altering the endometrium).” This means that Plan B can prevent a fertilized egg from attaching to a woman’s womb by causing the lining to thin. Plan B is thus denying the fertilized egg to obtain the nutrients it needs to survive.
And the issue isn't whether or not you support a woman's right to have an abortion, it is that if indeed what the above statement says is true, you are now FORCING pro-life employers to facilitate their employees abortions.
I feel abortion is murder, but agree that it isn't my choice to make. However I do see the problem with forcing a person who shares my belief to pay for insurance that will provide that pill to their employee. The government then is forcing them to essentially aid an employee in murdering their unborn child.