Rosalind, Viola and Portia are lead characters in plays by William Shakespeare, all of whom at some point disguise themselves as boys or men.
Rosalind is the heroine of As You Like It, who appears in disguise as the young page Ganymede;
Viola is the heroine of Twelfth Night, who appears in disguise as the young page Cesario;
Portia is the heroine of The Merchant of Venice, who appears in disguise as the young lawyer Balthasar.
The reference to the legitimate stage draws our attention to the fact that music hall theatre was considered inferior to, and less respectable than, more traditional forms of dramatic art. It also refers to the fact that music hall acts sometimes contravened laws of 'decency', and were sometimes banned on these grounds.