How does Portia describe the role of most Roman wives? How does her relationship with Brutus differ from the ordinary marriage?

Portia has a unique relationship with Brutus in that she expects the marriage to be a shared experience rather than a top down patriarchy. When Brutus refuses to confide in Portia, she insists that no secrets be kept in their marriage,

Dear my lord,
Make me acquainted with your cause of grief...
Within the bond of marriage, tell me, Brutus,
Is it excepted I should know no secrets
That appertain to you? Am I yourself
But, as it were, in sort or limitation,
To keep with you at meals, comfort your bed,
And talk to you sometimes? Dwell I but in the suburbs
Of your good pleasure? If it be no more,
Portia is Brutus' harlot, not his wife. Act 2

Portia does not want to be treated like a mere toy or harlot to be used when Brutus fancies her. She demands an equal role in his life and all his affairs.