In addition, Brutus is a good husband. He understands that their "enterprise" may not go according to plan. His unwillingness to immediately share the details of the conspiracy shows an honest concern for the well being of his wife. If she doesn't know anything, she can't be accused of helping to kill Caesar. Both Brutus and Portia have each other's well being in mind.
Brutus and Portia have a strong and loving relationship. Portia is naturally worried about Brutus, as she has observed a difference in his behavior. He is not sleeping well, his temper is different, he is absent-minded and even when he is with her, he is not "with" her. His mind is elsewhere and his distraction indicates to her, the observant wife, that something is wrong.
She tries to convince him to talk with her...she even goes so far as to chide him for not sharing his internal conflicts with her. She says she is "man" enough to handle it and that he should discuss matters with her in order to straighten them out in his head. She tells him in 2. 1 that she is strong and loyal to him and she deserves to know the truth. She threatens to give herself a voluntary wound in the thigh to prove that she can take the pain...both physically and emotionally...of whatever he has to say.
He responds with, "O ye gods, Render me worthy of this noble wife!" He tells her he will share with her his problem later, and they are interrupted by a knock at the door.
Portia sends Lucius to observe Brutus at the Senate on March 15, and orders him to come back and report to her. She is a good wife.