רמב"ן — על כן יעזב איש

Q — Rashi reads 'על כן יעזב איש' as the Torah's warrant for forbidding arayot to Bnei Noach, and 'והיו לבשר אחד' as referring to the child produced. What's wrong with this reading?

A — Ramban objects: if 'לבשר אחד' meant the child, animals would also be 'לבשר אחד' in their offspring — yet the pasuk frames this as something distinctive about ish. The chiddush is that the pasuk describes the unique human capacity for permanent devekut to one mate. Animals couple and disperse; man, because his isha was 'עצם מעצמיו ובשר מבשרו', clings to her ('היתה בחיקו כבשרו ויחפוץ בה להיותה תמיד עמו'). This natural devekut was 'הושם טבעו בתולדותיו' — implanted in his descendants. The pasuk is anthropology, not arayot legislation.