In some cultures girls are not allowed to mix with boys. Whether we think this is wrong or right, it happens. Fact.
Girlguiding was developed as a girl-only group, Not in order to exclude boys, but to include those girls who may otherwise be excluded of boys were present. If girls want to mix with boys, or if boys want to mix with girls then there is the scouting movement (or indeed almost any other activity) available to them.
With girlguiding any girl can go to any activity/meeting in any unit in any country and be assured of being in a girl-only environment.
This is from the girlguiding website:
In addition to providing a ‘girl only’ space for our young members, we are committed to ensuring our young members have female role models in leadership within Girlguiding. In order to support this, roles such as Unit Leader, can only be undertaken by female members.
Whilst some roles are restricted to only being held by women, adult males (18+) are welcome to join the charity in a variety of other volunteer roles.
I can't find anything about males of under 18 being admitted in any role. Therefore, I can understand why someone would question his involvement in this girl-only organisation.