Nobody cares if a man likes the idea of women-only classes. They're not for you. They're for women.
With some historical perspective, poo-pooing the idea of women-only classes becomes even more ridiculous. In my lifetime, women were banned from many karate and judo dojo. To turn around so quickly and say that women must train with men, not on their own, should set off alarms in your head.
Some women want to train with only women. Women-only karate clubs follow this concept and add women-specific self-defense concepts. There is even some evidence that single-gender school environments improve academic achievement.
BJJ brown belt Sally Arsenault has specific reasons to run women-only classes: they allow her women student athletes to progress faster, in a safer environment, without harassment.
I can’t say that if I could go back in time, I would choose to start BJJ in a women-only beginner class over a co-ed class, but I think I would strongly lean towards it. I had some shitty experiences as a brand-new white belt training with brand-new male white belts.
...Brand-new male white belts (BNMWBs) are dangerous to themselves and others. Some BNMWBs are clueless. They don’t understand that it’s better to try and match your athleticism, strength, and technical skill to your opponent’s during rolling in order to help each other get better. They just want to get that submission.
How many times has one of these fucking idiots injured me? I can’t count.
This aspect of size is so incredibly strong in sparring-based arts that many lighter men will only train with similarly-sized men. This is seen in boxing, where finding sparring partners of appropriate size is critical, as well as in grappling arts (for instance, Felipe Costa's lightweight-only group). Having a time set aside to train with people closer to your size, temperament, and background can be tremendous.
On several occasions my bottom was groped during rolling or drilling. I have been greeted with the term “slut,” [and] been bullied in front of others for show
You can pretend that this would never happen at your club, but an instructor can't be everywhere, and plenty of men are fine with men but inappropriate with women.
You and I might know martial arts as an inclusive, welcoming place for women. But the general public doesn't. Lots of people who don't train have a view of martial arts schools--yours, mine, anyone's--as a machismo-drenched chest-beating show of militarism and brutishness. Women-only classes and seminars can alleviate these barriers, thereby bringing more women into the practice:
Having a women’s only class, seminar, or open mat is a great way to introduce women into the world of martial arts, especially if they have had no prior experience in sports or martial arts. Starting something as awkward and hands-on as jiu jitsu, can be intimidating. It can be even more intimidating when you see guys bigger than you armbarring and choking each other out. However, seeing a class full of women training can be more inviting and put a beginner more at ease. This gateway also promotes growth.