In our Shari'ah, we do not have any prohibition or censuring of marriage of a Seyid/Seyidah to a non-Seyid/Seyidah. This is a cultural practice and personal preference of a parent.
One must bear in mind though that if one is to marry a Seyid/Seyidah, they must stress on a higher level of honouring and respecting them, due to their lineage to Ahlul Bayt (a.s.).
As this is a frequently asked question, please refer to other answers given on this:
And Allah knows best.