Nope. Women should not say yes, if they are not ready. Marriage readiness comes from realizing that both parties are mature, compassionate, and understanding enough to be able to create a lasting relationship. After all, marriage is a life-long commitment; hence, it should not be taken lightly. I do think that there should be mutual understanding and that both parties are sensitive enough to perceive whether both of them are ready for the commitment of marriage. Words uttered cannot be taken back even with the swiftest horse. And taking it back will not only pose conflict but worse may also jeopardize the relationship.
Saying no, on the other hand, doesn't necessarily mean that a woman is unsure of the person she's in a relationship with. Perhaps, she still has a lot of things she wants to do before actually settling down.