What is the main theme revolving around Portia and Nerrisa disguising as men and Jessica's disguise as a men as well to elope with Lorenzo?
Why did the women have to disguise as men? Was women dominance a theme shakespeare wanted to portray in the merchant of venice?
In the male dominated world these women lived in, to be taken seriously it was necessary to be a man. Had Portia and Neressia appeared as themselves in the court, they probably would not have been allowed to speak. Portia would not have been able to question Shylock but disguised as a man, no one questioned her skill or knowledge. As women, they were not free but as men they were.
As for Jessica, she woud not be noticed in the company of men if she was dressed as a male. As a woman, she would have been noticed.
Of course a more practical reason was that during the Elizabethan period, women were not allowed to act on stage and thus women's parts were played by young boys and men. They would have been much more comfortable dressed as boys and men.
It is evident from the beginning that Portia is a captive of her father's will. The caskets are his defense against greedy men who only wish to marry Portia for her money, which is considerable.
It is also clear that Jessica is also a captive. Perhaps Shylock is an over protective father considering how Jews were regarded by Venice's Christian society.
In that world, women submit to men.