Here is my query:

SELECT STRAIGHT_JOIN p.id FROM pool p LEFT JOIN sent s ON s.campid = 'YA1LGfh9' AND p.username = s.username WHERE p.gender = 'f' AND s.username IS NULL;

Here are my indexes:

mysql> show index from main.pool;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| pool  |          0 | PRIMARY  |            1 | id          | A         |     9326886 |     NULL | NULL   |      | BTREE      |         |
| pool  |          1 | username |            1 | username    | A         |     9326886 |     NULL | NULL   |      | BTREE      |         |
| pool  |          1 | source   |            1 | source      | A         |           2 |     NULL | NULL   |      | BTREE      |         |
| pool  |          1 | location |            1 | location    | A         |       38382 |     NULL | NULL   |      | BTREE      |         |
| pool  |          1 | pdex     |            1 | gender      | A         |           3 |     NULL | NULL   |      | BTREE      |         |
| pool  |          1 | pdex     |            2 | username    | A         |     9326886 |     NULL | NULL   |      | BTREE      |         |
| pool  |          1 | pdex     |            3 | id          | A         |     9326886 |     NULL | NULL   |      | BTREE      |         |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

mysql> show index from main.sent;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| sent  |          0 | PRIMARY  |            1 | primary_key | A         |         351 |     NULL | NULL   |      | BTREE      |         |
| sent  |          1 | username |            1 | username    | A         |         175 |     NULL | NULL   |      | BTREE      |         |
| sent  |          1 | sdex     |            1 | campid      | A         |           7 |     NULL | NULL   |      | BTREE      |         |
| sent  |          1 | sdex     |            2 | username    | A         |         351 |     NULL | NULL   |      | BTREE      |         |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+


Here is the EXPLAIN:

mysql> explain SELECT STRAIGHT_JOIN p.id FROM pool p LEFT JOIN sent s ON s.campid = 'YA1LGfh9' AND p.username = s.username WHERE p.gender = 'f' AND s.username IS NULL;
+----+-------------+-------+-------+---------------+------+---------+------+---------+--------------------------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows    | Extra                                |
+----+-------------+-------+-------+---------------+------+---------+------+---------+--------------------------------------+
|  1 | SIMPLE      | p     | index | pdex          | pdex | 161     | NULL | 9326886 | Using where; Using index             |
|  1 | SIMPLE      | s     | index | sdex          | sdex | 309     | NULL |     351 | Using where; Using index; Not exists |
+----+-------------+-------+-------+---------------+------+---------+------+---------+--------------------------------------+
2 rows in set (0.00 sec)