They quote this on their FAQ page: girlsfriendlysociety.org.uk/what-we-do/faqs/
Studies have shown that accessing single-sex space is important for girls and young women. A review by YWCA and Girlguiding UK found that girls and young women were enthusiastic about having time away from boys. They also found that single-sex spaces encouraged girls to take more risks, express themselves and develop their self-confidence. In mixed gender settings, boys tended to dominate the space which reduced girls’ confidence. A single-sex environment also helps girls and young women to think about and challenge gender stereotypes.
Are they genuinely single-sex (not gender)?