Racism is something that consumes your entire mindset. Sexual preference is just a tiny fraction of that mindset. Not being sexually attracted to someone because of their race doesn't cross over into being completely intolerant and carrying a hatred of that entire race. They're not the same thing.
And no, all people of any given race don't look the same, but many of them have similar features. As human beings, we take that complex and vast array of features and break them down into simplified groups. It's the way our minds work. Are you really implying that the person who made the ad should've said "Men with light skin, straight hair, and Spanish accents only?" Is our society really that politically correct at this point?
Lastly, let's look at the context of this scenario. We're talking about anonymous hookup sites here. Most people go to those with one direct goal in mind, and it's to hookup with an object of fantasy: a jock, an Asian person, a hairy person, a cross-dresser, a guy who's willing to pee on them, whatever that fantasy may be. Just because the person who puts up the ad turns down somebody who DOESN'T fulfill that fantasy for anonymous sex doesn't mean that they'd turn them down for normal sex all the time. Someone can put up an ad asking for only white people, but if Tyson Beckford showed up at their door five minutes later with a box of condoms and lube, they'd probably say yes.
So for the last time, sexual preference is not indicative of someone's overall racial attitude. I don't know why anyone would think it is.
While your post was eloquent, you're missing the larger structural problem - racial attitudes are socially constructed the same way our sexual attitudes are.
And I disagree - I think there are many men, when asking for only white men, would also turn down Tyson Beckford. And even the ones that are willing to fuck Tyson Beckford, would do so because it's Beckford, and not merely dismiss him because he's black. Seriously, there's a political group called "racists for Obama". When you see someone as an individual as opposed to a social group, you treat them differently. Many gay men will dismiss blacks off hand, even if they may not if they saw each black as an individual. But the fact that they do not is affirmation to racially-ingrained attitudes.
You don't hear the media ever calling a criminal as being "white", but when the criminal is black, you are sure that race is brought up. Because white people are unique snowflakes and everybody else are merely members of a race, right?
And you're claiming that anonymous sex != normal sex. Yet I see the "no X race" even on sites such as gaydar, which is both for people looking for hookups and dating. You're claiming it's "merely fantasy". I think it's much deeper than that.
The fact that people say "no X race" on anonymous NSA sex ads is simply because they are able to get away with it. They are able to be as shallow and reckless as much as they want because consequences don't matter in a NSA sex ad - they're looking for a one night stand, not a boyfriend. But who says these shallow preferences are thrown out the window when they're looking for a boyfriend?
They aren't. Just because a person doesn't state shallow preferences on a "serious ad" doesn't mean they don't exist. Minorities in America are not merely be denied anonymous sex in concern with particular "sexual fantasies" - they are being denied "serious" dating, and long-term relationships too.
Various economists have published papers on dating preferences in America. To say that race suddenly disappears when people are not looking for shallow one-night standers is simply misguided.