Pete Townsend’s personal history maybe.
From Wikipedia:
. The song was originally intended to be a part of a rock opera called ‘Quads’ which was to be set in the future where parents can choose the sex of their children. The idea was later scrapped, but this song survived and was later released as a single.
The song is about a family who “order” four girls, but a mistake is made and three girls and one boy are delivered instead. The boy dreams of partaking in sports and other boy-type activities, but his mother forces him to act like his sisters and refuses to believe the truth (Thereby the refrain “I’m a boy, I’m a boy, but my Mum won’t admit it...”.)