If the flood of Noah is to be taken literally, all men are descendants of Noah's 3 sons. Shem is the father of the Middle Easterners (and maybe the rest of the Asians). If the Arabs aren't Semites it would be hard to classify them. They don't appear to be Hamitic and they certainly are not of Japheth. Their temperament and culture seem to fit well with the Semitic peoples.