For the first time ever, the Health and Human Services secretary publicly overruled the Food and Drug Administration, refusing Wednesday to allow emergency contraceptives to be sold over the counter, including to young teenagers. The decision avoided what could have been a bruising political battle over parental control and contraception during a presidential election season. The contraceptive pill, called Plan B One-Step, has been available without a prescription to women 17 and older, but those 16 and younger have needed a prescription — and they still will because of the decision by the health secretary, Kathleen Sebelius.