Spock is half-human, and we see how badly he was treated on Vulcan, so, there seems to be a thread of racism in Vulcan society (even in Sarek from the movies). T'Pring was arranged for marriage to Spock by their parents. T'Pring's family are either not racist, a strong ally to Sarek, or probably both, to be willing to accept a half-breed into their family. Nothing in T'Pring's speech to Spock is even remotely racist as to why she did not want to get married to Spock. At least T'Pring was not a racist despite her other negative traits such as lust and greed.