It is notable that it is Juliet who first proposes the idea of marriage. Though still very young, even she knows that a woman in her position needs to protect herself, needs the security and respectability that marriage can bring.
Deep in her heart, Juliet may believe that Romeo's intentions are honorable. But she still needs an outward sign that Romeo's serious. Raising the prospect of marriage is very useful in this regard. It can be seen as almost a test of Romeo's intentions towards her, a way of making sure that Romeo really means what he says.
If Romeo is as deeply enamored of Juliet as he says he is, then Juliet is certain that he will have no problem with making their relationship official by agreeing to marry her. If Romeo's intentions are genuine, if he doesn't regard Juliet as just another infatuation, then he will have no hesitation in sending word the following day that he is ready to marry her:
If that thy bent of love be honorable,
Thy purpose marriage, send me word tomorrow. (II, ii, 143–44)
If he should do so, then that will put their relationship on a much stronger footing. Though it will still be a deeply intense, passionate romance, it will also have the firm foundation of a commitment to engage in the sacrament of holy matrimony.