I think that if a martial arts teacher wants to separate the sexes, he has a perfect right to.
On the other hand, if a martial arts class has always mixed the sexes, with the agreement of the teacher and the students, and then a Muslim comes along and demands they be separated, that’s a whole other matter.
I’d tell the Muslim that if he doesn’t like the way classes are run, he can go somewhere else.
He would sue and problem win. Islam trumps all it seems..
he didn’t have to fight them or anything, but having his own section of the place seems out of bounds