You may be interested in this previous answer of mine, I've read that it was incredibly normal for brides to be pregnant in the 18th century, but in Pride and Prejudice a couple's implied sexual contact prior to their marriage is a huge scandal. Is this indicative of a moral class divide, or were 18th century values just not representative of reality? This gets to the real problem the characters had with Lydia's behavior - not that she chose to get married far from home, but that before Mr. Darcy's intervention she was going to have had sex (or be thought to have had sex, which was just as bad) without any real expectation of marriage. That is what "elopement" meant in the period.

The good news quickly spread through the house, and with proportionate speed through the neighbourhood. It was borne in the latter with decent philosophy. To be sure it would have been more for the advantage of conversation, had Miss Lydia Bennet come upon the town; or, as the happiest alternative, been secluded from the world, in some distant farm house.

The assumption everyone was making was that she'd just run off to have sex, which would have meant that either she'd return to her family and have to be stuck far away somewhere by herself to mitigate the shame, or she wouldn't return but be forced to/choose to fend for herself in London as a prostitute ("come upon the town").