Technology

The hippocratic oath, in this case. Medicine is all about risk management, the worse the "disease," the more tolerant we are of side effects for the cure. Pregnancy and birth are still pretty traumatic events that, while much safer than they used to be, are still dangerous. Female BC just has to be less risky than that. Male BC on the other hand, has to be as low the risk for a man impregnating a woman, which is to say, almost zero. Pretty much any negative side effect is worse than that, so it's very difficult to pass. I would gladly take one with comparable side effects to female BC, but sometimes unflinching ethics are inconvenient. Better than the alternative, but still.