We now know that Marianne and Willoughby were never engaged. I always think it is a good time to review the fact that they have between them broken most/all the rules of courtship which include the list below. Couples were prohibited from...

1. driving in carriages/walking alone together

2. giving/accepting gifts

3. engaging in written correspondence

4. intimate touching

5. using each other's Christian names

In my opinion Mrs. Dashwood was remiss in not insisting on knowing the particulars of Marianne and Willouhby's relationship. Open engagements were looked on as necessary to protect the the couple, particularly the woman I think. Secret engagements were bad enough but the lack of an engagement with all the appearance of it was worse I would imagine. If/When it ever got out that she had never been engaged, her reputation could be/have been called into question. Having watched Marianne's behavior with Willoughby, it gives us a basis for how the Eliza seduction probably took place. I wonder if Marianne was at all worried about the consequences that could befall her and by association, her sisters. Probably not at this point. She seems to be so steeped in her wretchedness right now. Her point of view is still very self-centered.