We've all heard it before:
"I'm not racist, I just wouldn't date a [insert race/ethnicity] person."
Firstly, a piece of advice: If you have to begin your sentence by clarifying that you are, in fact, not racist, it's a fairly good indicator that you need to reconsider what you're about to say.
By making, or agreeing with the statement that you "just wouldn't" date members of a certain ethnic or racial group, you are making an absurd generalisation about that group: that their superficial qualities would make it impossible for you to form an emotional, romantic or sexual connection.
In case anyone hasn't told you: this makes you prejudiced.
Despite this, racial or ethnic "preferences" are repeatedly brushed aside as a valid and unalterable aspect of a person's fundamental individuality, and questioning them threatens individuals' personal expression with restrictive "political correctness."
Let's get this straight: you are not oppressed for being called out on stereotyping others.
I'll put it this way: my cousin (20, straight, overwhelmingly middle-class) finds blue eyes attractive; a few of his past partners have had blue eyes. However, if a girl with brown eyes started flirting with him... "Would I turn her down? Based on her eye colour? Are you crazy?!"
Most people would agree with my cousin - not dating someone on the basis of the colour of their eyes sounds ridiculous, right? No rational human being would ACTUALLY do that. Even if you're not that rational, you understand that the slight edge in perceived physical attractiveness that the blue eyes holds for him could more than easily be compensated for by other traits he finds attractive - a sense of humour? Empathy? A shared passion for indie-rock of the feminist persuasion? (A girl can dream...)
Well, the same goes for racial/ethnic traits, and most crucially, for skin colour. Perhaps your potential partner's skin is a different colour to yours. Or perhaps it's the same. Whatever your supposed "preference," you have absolutely no idea about their personality - they could be a gun-toting drug lord; they could be the next Gandhi. But, hopefully, you recognise that skin colour (along with other biased and oppressive racial stereotypes) has no effect on their potential romantic compatibility with you.
To clarify, inclusionary racial preferences can be racist as well. Claiming that you are only interested in dating members of a specific race is equally insulting, because it defines someone by their ethnicity (or your perception of it.) Suggesting that someone's racial background gives them more desirable qualities reflects the damaging histories of colonialism and the fetishisation of people of colour that went along with it.
Basically, if someone's skin colour alone is enough to make you attracted or not attracted to them, you've got some serious life-evaluation to be getting on with.
Finding someone attractive, or unattractive, based on their physical features is not racist (although it is rather superficial) but assuming that those physical features carry some sort of meaning or cultural significance most definitely is. You can't possibly claim that you know for a fact that every single person of a given race or ethnicity has no chance of falling in love with you based entirely on physical appearance.
Many people use the "exposure" excuse as an attempt to justify their racial preferences, claiming that they have very little experience of interacting with a certain racial group, and therefore cannot find them attractive. By using this argument, you are essentially holding your hands up and admitting your cultural bias. You have never had the opportunity to try and form relationships with anyone from that ethnic group, so there is absolutely no way that you can "know" that you won't be attracted to them or enjoy their company. You are allowing cultural stereotypes to control your love life.
Closely related to this argument is the idea that the cultures associated with people of certain races and ethnicities are incompatible with others, leading to the conclusion that the two cultures are too contradictory to even bother trying to engage in a relationship.
Other people try to bypass the whole race issue by invoking a personality trait such as "they're so lazy" or "they only think about themselves" that is assumed to be typical of that particular ethnic group. This kind of implied universalism is biased and narrow-minded. Basically, it's really, really racist. But, this particular excuse is also dangerous - it relieves the racist individual of their personal responsibility for their opinions.
This is not to say that you should feel obligated to be with a person simply for the sake of inclusivity. Anyone (and everyone) can have undesirable aspects, however it is completely illogical to assume that those are ethnically specific.
Racial and ethnic preferences reduce people to their ethnicity, and insinuate that skin colour or perceived traits are an important enough factor to negate everything else that a person has to offer.
You cannot know whether or not you are compatible with someone unless you actually get to know them.
Abandon your cultural biases and racial stereotypes. It'll make dating a whole lot easier. I promise.