In the carriage, Mr. Gardiner wonders if Jane might be right in hoping for the best: Wickham knows Lydia has no money and stands to lose his reputation with his regiment, so what else could he be after but marriage? Elizabeth assures them that Wickham is an awful person, capable of anything.
Elizabeth knows from Darcy's story that Wickham has no morals and is therefore capable of anything. Only marrying him can save Lydia's reputation, and that of her entire family.
At Longbourn, Mrs. Bennet is holed up in her room, frantic with nervousness, and blames Colonel Foster for not watching over Lydia. She tells Mr. Gardiner to make Lydia and Wickham marry when they are found—and to make Lydia consult her about finding the best deals on wedding dresses.
Elizabeth and Jane dissect the situation. They are relieved that apparently Lydia did think she was getting married, which they can tell from a note she left for Mrs. Forster signed "Lydia Wickham."
If Lydia had not intended to marry, and was just after a sexual fling, she would be seen an indecent "fallen woman."