The Qur'an says: "The Prophet is closer to the believers than their own selves and his wives are their mothers" (33:6). What has been established by the Qur'an is the prohibition for men to marry the wives of the Prophet. The prohibition for a sayyedah to marry a non-sayyid is a wrong assumption and analogy. We don't find any instruction as such in the sunna of the Prophet and the teachings of the Imams after him. Rather what we find is the emphasis in choosing a religious and pious soulmate for a successful marriage leading to happiness in this world and everlasting bliss in the next one insh'Allah.
With prayers for your success.