For the male patients that have a normal prothrombin time, what is their average prothrombin time?
male refers to Sex = 'M'; normal prothrombin time refer to PT < 14; average prothrombin time = AVG(PT);
SELECT AVG(T2.PT) FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE T2.PT < 14 AND T1.SEX = 'M'