Quote 1
PORTIA
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 your self
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. (2.1.275-276; 302-310)
When Brutus refuses to confide in his wife, Portia takes issue with his secrecy: as a married couple, she says, they should have no secrets. In other words, Portia is sick and tired of being excluded from her husband's world just because she's a woman. She also suggests that, when Brutus keeps things from her, he's treating her like a "harlot, not his wife."
Quote 2
PORTIA
I grant I am a woman; but withal
A woman well-reputed, Cato's daughter.
Think you I am no stronger than my sex,
Being so father'd and so husbanded?
Tell me your counsels; I will not disclose 'em.
I have made strong proof of my constancy,
Giving myself a voluntary wound
Here, in the thigh. Can I bear that with patience.
And not my husband's secrets? (2.1.215-325)
Yikes! Portia seems to buy into the all-too-common idea that women are weaker than men. Here she says she knows she's just a girl but reasons that, since she's the daughter and wife of two really awesome men, that makes her better than the average woman. To prove her point, she stabs herself in the thigh without flinching and demands that her husband treat her with more respect.
History Snack: When Portia says she knows she's just "a woman" but she also thinks she's "stronger" and more constant (i.e., masculine) than most, she sounds a lot like Queen Elizabeth I (Shakespeare's monarch) who famously said "I know I have the body but of a weak and feeble woman; but I have the heart and stomach of a king" ("Speech to the Troops at Tilbury," 1588). Queen Elizabeth I, like Portia, buys into the idea that women are generally weaker than men but presents herself as the exception to the rule.
Quote 3
PORTIA
I must go in. [Aside] Ay me, how weak a thing
The heart of woman is! (2.4.45-46)
OK, OK, we get it. The characters in Julius Caesar (including Portia, one of only two female characters in the play) think women are "weak."