Heard from my Skype chavrusa Rav C.S. Shlita....
Rashi teaches in this weeks parsha that Eisav had bi-ah with a נערה המאורסה. Tosfos in Sanhedrin [57b] notes that Bnei Noach are not commanded not to have relations with a נערה המאורסה - only with a בעולת בעל and therefore although Eisav didn't do an aveirah strictly speaking, it was still disgusting.
But Tosfos never address a more basic question - Where do we find a נערה המאורסה before Matan Torah. The whole notion of אירוסין started after the giving of the Torah - no??
So in Talmudic parlance - היכי משכחת לה נערה המאורסה לפני מתן תורה??
