When searching for a school to attend, you need to find a place that suits your learning style and social needs. Some people may feel more comfortable with people of their own gender. There are people who do not feel comfortable around the opposite sex for various reasons. If a same sex school will promote more learning, it needs to be allowed.
In response to the opponent, it is not forced upon people to go to a same sex school. This should be an alternative to public school. If this is the difference that can make a student succeed, it needs to be legal.
It doesn't matter if the results of single-sex education are positive because the world isn't divided into girl and boy areas. All genders are going to interact with each other at work and in stores and every other place in the world.
And what about the children who identify as another gender? Would a girl born as a boy be forced to go to a boy school and vice versa? What about the ones who fit in neither category, somewhere between girl and boy or not even on the spectrum?
Would the education be equal? Would a girl and boy have the same chances and opportunities as the other? Or would they focus on what society assumes they'd like/be better at?
I've read about sex-segregated schools. They would employ unfair assumptions about the differences in the ways boys and girls learn that were often detrimental to the students. This sort of schooling will be biased to keep girls away from math and science.
If we want to tailor education to help students learn better it should be based on tests and observations of individual students in order to sort them, not based on gender. Even if male students tend more often to learn a certain way, then it is just a tendancy not absolute. So it is better to examine the students themselves and sort them accordingly.