1  with t1 as (select 1 ID from dual union all
  2                select 2 from dual union all
  3                select 3 from dual union all
  4                select 6 from dual),
  5       t2 as (select 2 ID, 3 Pair from dual union all
  6                select 3, 2 from dual union all
  7                select 5, 6 from dual union all
  8                select 6, 5 from dual)
  9  select * from t1 where
 10  not exists (select 1 from t2,t1 tt
 11              where tt.id>=t1.id and (t1.id,tt.id) in ((t2.id,t2.pair)))
 12* order by 1
SQL> /

        ID
----------
         1
         3
         6