The first discussion of marriage involving Juliet is in Act 1 Scene 3. It is Lady Capulet who suggests this notion to her daughter, and asks her to consider Paris as an appropriate match. Juliet says she had not contemplated marriage. The discussion involves Juliet, her mother and her nurse, emphasizing how important the nurse is to life of the Capulet family; and Juliet in particular.
It is in Act II scene ii when, bewitched by the intensity of her feelings for Romeo, Juliet asks him if he is true in his intentions, and is committed to marrying her.
If that thy bent of love be honourable,
Thy purpose marriage, send me word to-morrow,
It is likely that the notion would not have occurred to her so readily if she had not been discussing marriage with her mother and the nurse earlier in the evening.
Juliet is looking for affirmation that Romeo’s purpose is the same as hers, and that his passionate words are based on real emotion. The suggestion that they marry is her way of testing his motives. Both become excited by their daring plan.