Since Spock was free after his marriage to T'Pring fell through, presumably he would have been free to choose another woman. After all, Sarek had been free to choose Amanda. What if Spock had chosen to marry a human? Would Sarek have welcomed her into the family, or would he have snubbed her and Spock? Logic or no logic, Vulcans can look down on people and can hold grudges. Look at the way Sarek held a grudge against Spock for eighteen years just because Spock had joined Starfleet. Imagine if he married a human. Never mind that Sarek had married a human himself. "Do as I say, not as I do" seems to apply to Vulcans as well as humans.