I am an ASA instructor, I am male, and I am in the USA (I understand that the cultural mores are different in other countries). I have never had a gender specific issue with either a male or female student. Because I am an ASA instructor, I have met and worked with a few female instructors (both ASA and US/Sailing), some of whom are great instructors, and some of whom I would not want to take a course with, let alone have them teach.
I once had a female student shut down on the morning of the first day of ASA 101. She was the female half of a married couple. After her initial turn at the helm, she refused to take the helm again and as a result she did not pass the class. To be fair to her, she was scared. Her turn at the helm consisted of my helping her in bringing the boat back to the marina where we started so that I could reef the main. Winds had picked up to about 18kts from under 10 when we left the mooring. Reefing is normaly covered later in the class. However, I believe that the main issue in this case was that the sailing dream was his, and she was attending the class to appease him. I believe that she shut down because she was terrified of failing in the eyes of her spouse.
She was able to act this way because her spouse was on the boat with her. In effect, he was her crutch. If she had either been with a group of friends or people that she did not know, she would not have shut down as she did. If not for his presence on the boat, she would have either risen to the challenge of taking the helm again, or quit the class because sailing was not for her. I have experienced similar behavior when trying to teach (ex)girlfriends how to (snow) ski.
Whenever possible I prefer to split "couples" into seperate boats for ASA 101.
Regarding the OP; Did your wife fail the written or the practical portion of ASA 104? If she failed the written, she needs to study the book or spend more time aboard a boat. If she failed the practical, I suggest that she does not need to take a course with a female instructor, she simply needs to take the course without you.
For those reading this other than the OP; In ASA 103 students are taught to manuver a 30+ foot boat, dock, and anchor. They are also introduced to the mechanical systems on the boat and COLREGS. In ASA 104 students are taught how to provision, more about systems and COLREGS, and how to navigate with a chart. 104 classes also leave the marina overnight. See https://asa.com/certifications/
From my perspective 103 is a harder course to teach, and pass, than 104.