1. Mix together grapefruit juice, olive oil, marjoram, salt, and pepper, in a shallow glass dish. Add fish. Turn once to coat both sides, then cover dish and refrigerate for 1 to 2 hours, turning once or twice.
2. Set grill about 4 to 6 inches above coals and preheat barbecue or gas grill. Grill for 10 to 12 minutes, turning once and brushing twice with remaining marinade, until steaks are barely opaque in thickest part. Arrange on a platter, scatter grapefruit sections around steaks.