What does Judges 21:7 mean?

First, Israel slaughters nearly the entire tribe of Benjamin (Judges 20:47–48) for defending their city of Gibeah (Judges 20:12–13), whose people were guilty of heinous crimes against the Lord. They seem relieved that several hundred Benjaminites escaped the carnage and were now hiding in nearby caves. They don't want the tribe of Benjamin to become extinct. They don't want the circle of the tribes of Israel to be broken.

However, before the fighting with Benjamin began, all eleven tribes took an oath not to marry their daughters to any men of Benjamin. They do not even consider breaking this oath, likely because it was well known the Lord would judge those who broke sacred oaths (Deuteronomy 21:21–23). Since all the Benjaminite women and children have been killed and marrying Canaanite women is also not allowed (Deuteronomy 7:1–5), the survivors of Benjamin have no hope to rebuild their tribe.

As it happens, the leaders of the Israelites have an idea for how they can find genuine Israelite wives for the men of Benjamin without breaking their oath (Judges 21:14).