This claim is based on cultural view in the sub continent and among some believers who have influence from that culture. In Islam, we do not have any discrimination between believers from any background, language, ethnicity, etc.
Those who claim that Sayyeda is forbidden from marrying a Non Sayyed need to present evidence and they will never be able to present any authentic evidence as our authentic narrations from the Prophet (SAWA) and Ahlul Bayt (AS) confirm that any believer man can marry any believer woman whether Sayyed or non Sayyed. Our leading Ulama and Maraaji' of Taqleed who are Sayyeds have given their daughters in marriage to non Sayyed believer e.g.
1. Ayatullah Sayyed Mohsin Al-Hakeem (AR).
2- Ayatullah Sayyed Abul Qasim Al-Khoei (AR).
3. Ayatullah Sayyed Ruhullah Al-Khumaini (AR).
4. Ayatullah Sayyed Mohammad Redha Gulpaigani (AR).
5. Ayatullah Sayyed Mohammad Saeed Al-Hakeem.
Cultural point of view should not overcome religious teachings. The damages of creating such restrictions will make thousands of our daughters suffer and it can ruin life of some of them. No doubt, those who caused this suffering will be responsible in this life and hereafter.
We finally tell them to ask the Marji' of Taqleed whom they follow who are the most knowledgeable about the teachings of the Prophet (SAWA) and Ahlul Bayt (AS).