It is against the teachings of the Prophet (SAWA) and Ahlul Bayt (AS) to create a condition in marriage which was never put by Islam. Caste condition is simply un-Islamic .
Delaying the marriage of a female who needs marriage, for no real Islamic valid reason is a sinful act.
The Infallible Imams were from Bani Hashim yet they got married to pious ladies out of Bani Hashim and even out of their language and culture. There is no question of caste in marriage in the teachings of Ahlul Bayt (AS).
The authetic Prophetic Hadeeth says very clearly : If marriage proposal comes to you from a person whom you accept his faith and manners, you should accept it, otherwise the earth will be corrupted with Dispute and wide spread evil.
إذا جاءكم ٬من ترضون دينه وخلقه فزوجوه ، إن لا تفعلوه تكن فتنة في الأرض وفساد كبير.