The thing is that publicly accessible single sex provisions are separated by sex category of people. In the provision of these services and facilities, it is not considered a reasonable expectation to have 100% protection from harm. Only the expectation to reduce risk of harm to the minimum manageable.
Hence it is not considered reasonable for a female person to expect to be protected from potential attack from another female person. Because it is considered low risk and the risk of injury from a female on female attack is also lower than an attack from a male person. Usually it is greatly lower than from a post pubescent male.
If you would like the now numerous studies that should that in general a male person who has experienced any part of male puberty is considerably stronger than female people in general. And the Olympic committee also ruled that ALL male people with transgender identities are excluded from female sporting events based on physical advantage due to modelling that was done. Meaning even those male people who have had puberty suppressed are considered to have an advantage.
Hence all male people above the age of about 8-10 are excluded from female single sexual provisions.