All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There is no evidence in the Quran or Hadeeth (narrations) that the Asyaad (who belong to the lineage of the Prophet, sallallaahu ‘alayhi wa sallam) should not marry except from among their own family. This idea did not exist in Ahlul-Bayt (those who belonged to the lineage of the Prophet, sallallaahu ‘alayhi wa sallam) during the life of the Prophet. The Prophet, sallallaahu ‘alayhi wa sallam, married his daughters to those who were not from Ahlul-Bayt, and ‘Ali ibn Abi Taalib also did the same.
This baseless opinion leads to injustice to the women of Ahlul-Bayt, as they become spinsters without marrying anyone on the basis of the above false claims. If a religious and good-natured person proposes to marry a girl from Ahlul-Bayt, then they should not refuse him on the basis that he is not from Ahlul-Bayt. By doing so, they are actually opposing the Prophet, sallallaahu ‘alayhi wa sallam, and his family members, and are also behaving unjustly with their daughters. The Prophet, sallallaahu ‘alayhi wa sallam, said: "If someone with whose religion and character you are satisfied asks for your daughter in marriage accede to his request; if you do not do so there will be corruption and great evil on the earth." [Abu Daawood and An-Nasaa'i]
Allaah Knows best.