That's pretty much the very definition of racism. Aesthetic preferences are one thing, but specifically tying preferences to "inherent" racial qualities is definitely racist on some level. Example:
1. "I don't like girls with frizzy hair." Not racist.
2. "I don't like black girls." Definitely racist.
3. "I don't like black girls because of their frizzy hair." May not be worth grabbing the torches and pitchforks for, but it is still racist by a baseline standard.
For one, not only are you making assumptions about an entire race and likely tying projection and personal experience to it unnecessarily, but you're also using small numbers of people as spokespeople or representatives for an entire race, aka stereotyping. Even if you may not consider it offensive or derogatory or whatever in the way that actively subjugating someone to prejudice because of race is, it's absolutely a racist thing to do, even if subtle. "I normally don't like spanish guys, but you're pretty cute," "you're so polite for a Scotsman," "this is the longest I've ever been with a black guy," etc. are all no-nos. You're absolutely allowed to have preferences, for example wanting someone with red hair or freckles or glasses or even being tan, but the second you actually tie those qualities to a race, positively or negatively, it's absolutely racially-charged and by extension racism.
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I'll just call it generalizing instead of racism. If on a dating site you write [I]"I'm not into redheads"[/I], it doesn't mean that there isn't a redheaded person in the world that you would like. It's just that they're rather rare, and on a dating app/site you try to select and seperate who you're most likely into. Make the search for someone you do like as easy as possible. And that means eliminating the "long shots". You do it both for yourself and for those viewing your profile.
It's shallow as hell, and the selecting can be done with such broad strokes that it is just downright silly, but whatever. We're talking dating profiles here. the vast majority of hook ups are based on looks alone anyway, and I'm of the belief that you don't have much of a control over who and what you find attractive, so "not being into" something (whether it's a skin color, hair color, body type, ...) doens't necessarily mean a judgement on people, other then, well, attractiveness.
Doesn't change the fact that [I]"no xxx plz"[/I] in profiles gives off major douche vibes though.