According to the Midrash, the Benei Yisrael in Bamidbar 11 wished to have illicit relations with close relatives. In response, HaShem punished them with death via eating quail.
Why was this an appropriate punishment for this particular sin? In general, HaShem punishes measure for measure1. What was the measure for measure in this case?
Note that I am asking specifically according to the Midrash's understanding of this incident. For the purposes of this question, I do not care how anyone else understands what the sin was.
1Shabbos 105b derives this principle from Yeshaya 27:8, and Sanhedrin 90a derives this from Malachim Beis 7:1,2,20