Plan B consists of 2 pills that are essentially high doses of regular birth control pills, which require both a medical prescription and an ACOG recommended yearly gynecological exam. Plan B works by preventing ovulation or by prohibiting the implantation of a fertilized egg. Consequently, Plan B can act as an abortifacient.
The FRC opposes Plan B because it might prevent a fertilized egg from successfully attaching to the uterus. This begs the question: if a woman with endometriosis, has intercourse, is she an abortionist? With a condition like endometriosis, a woman could become pregnant (egg meets sperm) but could not carry a child to term (inhospitable uterus). The FRC, then, would demand such women not engage in sex, because sex, for her, would be murder.
Or something like that. I know there's a sincere belief that life begins at the moment sperm reaches egg. To the extent I'm making fun of that belief, I regret it, but I really don't understand why people would oppose a drug that probably prevents an unwanted pregnancy but might prevent a pregnancy from developing beyond a single cell.