Not sure......worked for me....maybe you need to be nicer to your ASE 
......works with sp122 - perhaps 121 had a regression????
1>
2> select @@version
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Adaptive Server Enterprise/15.7/EBF 22777 SMP SP122 /P/X64/Windows Server/ase157sp12x/3662/64-bit/OPT/Sat Apr 19 01:26:01 2014
(1 row affected)
1>
2> exec sp_displaylogin
Suid: 1
Loginame: sa
Fullname:
Default Database: master
Default Language:
Auto Login Script:
Configured Authorization:
sa_role (default ON)
sso_role (default ON)
oper_role (default ON)
sybase_ts_role (default ON)
replication_role (default ON)
mon_role (default ON)
sa_serverprivs_role (default ON)
Locked: NO
...
(return status = 0)
1>
2> use tempdb
1> create table t1 (a int, b int)
1> alter table t1 add primary key (a)
1> create table t2 (a int references t1(a), c int)
1> --set disable_ri_check on
1> insert t2 values (1,5)
Msg 546, Level 16, State 1:
Server 'PHILLY_ASE', Line 1:
Foreign key constraint violation occurred, dbname = 'tempdb', table name = 't2', constraint name = 't2_a_878241064'.
Command has been aborted.
(0 rows affected)
1>
2> drop table t2
1> drop table t1
....
1>
2> select @@version
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Adaptive Server Enterprise/15.7/EBF 22777 SMP SP122 /P/X64/Windows Server/ase157sp12x/3662/64-bit/OPT/Sat Apr 19 01:26:01 2014
(1 row affected)
1>
2> exec sp_displaylogin
Suid: 1
Loginame: sa
Fullname:
Default Database: master
Default Language:
Auto Login Script:
Configured Authorization:
sa_role (default ON)
sso_role (default ON)
oper_role (default ON)
sybase_ts_role (default ON)
replication_role (default ON)
mon_role (default ON)
sa_serverprivs_role (default ON)
Locked: NO
...
(return status = 0)
1>
2> use tempdb
1> create table t1 (a int, b int)
1> alter table t1 add primary key (a)
1> create table t2 (a int references t1(a), c int)
1> set disable_ri_check on
1> insert t2 values (1,5)
(1 row affected)
1>
2> drop table t2
1> drop table t1