What is it doing in Hilchos Lulav? Answer: To tell you that this Simcha was there to help us start a new clean year, without last year’s Aveiros; of which Arayos is always the example. (When the Rambam speaks about total doing Teshuva, the example he always gives is: בְאוֹתוֹ מָּקוֹם, וּבְאוֹתָּהּ אִשָּה עיין הלכות תשובה ב א, ומקורו מגמ' יומא פו ב refraining from the same sin in the same situation, etc.) (And when Chazal say “Aveira” Stam, it always refers to Arayos. (Maharal, Gr”a, etc.)
This is why the Rambam says that Amei HaAretz (the unlearned) were not directly involved in Simchas Bais HaSho'eva; only the elder Talmidei Chachomim, Roshei Yeshivos, and the heads of Sanhedrin. They were the focus, because the whole Simchas Bais HaSho'eva was an example of how to start the year and succeed against the Yetzer Hara with Simchas HaTorah. We can suggest that this is exactly why Noach started off with planting grapes for wine.
