Whether negi’ah is assur mi’doraisa, mi’d’rabbanan, or even muttar, is a machlokes amora’im in Shabbos (13a) and Avodah Zarah (17a).
Assuming it is assur, and it was mentioned that the Rambam paskens it is d’oraisa, it is a fence put in place to distance one from the actual act of giluy arayos. (Yes there are fences that are d’oraisa. The classic example is the prohibition of a nazir to eat grapes.)
Covering hair, IMHO, has nothing to do with arayos. I once wrote an essay on this and I had a number of problems trying to say it is indeed because of ervah,* and the truth is you wont find me a single rishon who explains it this way.
And by the way, the Rambam who learns that negi’ah is d’oraisa, understands that covering hair is only d’rabbanan. The Terumas Hadeshen (1:242) writes: ???? ????”? ????? ??? ???? ???? ??? ?????? ??????.
*???? ???? ?? ??? ????, ???? ??? ??? ??????, ????? ??? ???? ???? ????? ????? ???. ??????? ??? ???? ??? ???? ????? ?? ???? ??? ???? ????, ??”? ???? ???? ?????????? ???? ???? ???, ??? ??????? ??????. ???? ???”? ?????? ???? ?? ?? ????? ???? ???? ??? ??? ???? ???? ???, ??? ????? ?? ??? ???? ??? ???? ???? ??? ????? ??? ?? ????. ???? ??”? ???? ?? ????? ?? ??? ????? ??? ???? ???? ????? ???? ??? ???????? ???? ??? ???????. ??? ???? ????? ???? ????? ???? ????