select class_id
  from t_class t
 group by class_id
having sum(decode(t.sex, 'male', 1, 'female', -1, 0)) = 0;
select * from t_class;

select male.cid, female.cid
  from (select t1.class_id cid, count(1) total
          from t_class t1
         where t1.sex = 'male'
         group by t1.class_id) male,
       (select t2.class_id cid, count(1) total
          from t_class t2
         where t2.sex = 'female'
         group by t2.class_id) female
 where male.total = female.total
   and male.cid = female.cid;