The answer is:
Brutus wants to keep Portia out of the conspiracy to protect her.
In Act 2, Scene 1 from Shakespeare's "Julius Caesar," Portia confronts Brutus about his unusual demeanor and demands that he tells her what has troubled him. Brutus, who has been planning Caesar's murder and is meeting the conspirators, asks her to leave but comforts her by promising her that he will later tell her what the matter is.