Quranic revelation explicitly exempted Muhammad alone from marriage restrictions.

Evidence

Muhammad said that God spoke to him and said: ‘We allow thee thy wives, to whom thou shouldst give dowries, and all thy slaves whom God gave thee, and the daughters of thy paternal uncle and of thy paternal aunt, and the daughters of thy maternal uncle and thy maternal aunt, who have followed thee. And every believing woman, if she offers her body or herself to the Prophet, and if the Prophet desires to lie with her, let it be permitted, but only to thee and not to the other believers.’ A few words later (he goes on), ‘Thou shalt give hope to whom thou pleasest, that is, of those who otter themselves to thee, and thou shalt receive whom thou pleasest, and, if thou desirest, one whom thou hast sent away; and it is not a sin for thee.’ — Norman Daniel, Islam and the West, 564