Yes, I would definitely care, and it would be a deal breaker. I am not ashamed to admit it. I believe in equal rights and opportunities for everyone, but I don't believe in racial mxing.
And to all those who claim that only hillbillies or small minded mutants feel this way, well, embrace your new age mumbo jumbo if you like; I know that I won't be able to change your minds. Just know this: no one is going to give you a medal for being a white person that dates other races in order to be politically correct. If you really want to, then fine, but don't pretend like the sun is somehow shining out your backside because you do, and that other white people who don't want to, are somehow inferior creatures of the night. We are still allowed to have freedom of choice and freedom of expression in today's time, and that means that a white person should be able to be honest about his preferences in dating without being assaulted and harassed for being displeased with a girl's inter-racial dating track record. It does not even make him a racist, just culturally conscious and conservative.
He might view a white girl that has done this as being just a tad too liberal and indisciplined to have made such a bold move. In other words, they just won't be compatible. Whitenes and the features of white people - brown, red and blond hair, green and blue eyes, etc. are recessive and it might concern a man that the hite girl he is dating was at some point willing to not pass on those features. It is his right to like whiteness that much if he does. She will then be better off to be with a white man who doesn't care, but this does not make the other white guy a bad person. If it does, then why is someone who tries to conserve a certain threatened language from going extinct not considered as a hater of other languages?
Also keep in mind that us white people are not particularly blessed when it comes to penis size, with only small percentage having above 7" penises. So you might think you are this wonderful, open minded white guy, but when you make love to your white girl she is going to subconsciously compare you to the 12" African American lovers she had slept with in the past and oh boy, will you be coming up short in that department. Don't be surprised if that's the reason she leaves you. This is only logical and natural. They say size doesn't matter, but that's just women trying to be PC. In the back of their minds they will resent you for being inferior. Not that I have had personal experience with this matter (: it's just pure common sense.