Honestly, if you can't understand or articulate why you prefer something.. how do you know you're not just prejudiced?
So, you don't like to date people of particular ethnicities (or you prefer to date people of particular ethnicities above others) why? Like, why? Is it because you have a stereotypical concept of what being of a certain race "means" (i.e. you're looking for the submissive asian girl fantasy, or dating black guys feels transgressive). Is it because you've have bad experiences dating within some ethnic groups (like, you're an Asian woman and whenever you date white guys you feel like you become their "Asian girlfriend"). Do you find the facial appearance of some ethnic groups more beautiful or aesthetically appealing than others, and if so, why?
See, here's the thing.. you can be a racist without being hateful, or even without really being able to help it. If you only date white people because you find white faces the most beautiful, it may well be that you've absorbed a cultural code of beauty which equates whiteness and beauty, and that's not just a "personal preference" because it's replicated across society. Of course, knowing that you can't just shrug it off, but at least you're aware..
Other preferences are often a bit more nuanced. Obviously, most people are either gay or straight, and 90% of the time their gender preference is pretty unambiguous. But if you meet someone and you're attracted to them, and then you find out they're trans or they just look really gender non-conforming and that changes things for you, maybe you should ask yourself why? Like, sure, some people are viscerally disgusted by some genitals (although again, why) or would feel very uncomfortable having sex with someone without the genitals they prefer, that's how sexual orientation works for some people, but if the answer is that you now see that person as "a man" (or "a woman") and the mere thought of touching someone of the "wrong" sex grosses you out, then it's worth asking yourself what's going on there? If you feel disgusted at yourself because being attracted to this person makes you anxious that you're gay (or not gay) then do you have some kind of internalized homophobia or insecurity? In the case of trans people, is the problem that you're unable to see them as authentic? Are you worried that dating someone gender non-conforming will open you up to abuse or ridicule from others, and that scares you?
Again, you don't need to hate people to have absorbed prejudiced attitudes. We all absorb a few, because we live in a very prejudiced society. In general, people are very willing to tolerate prejudice when it comes to dating because we recognize that to a certain extent it's involuntary, but that doesn't mean you're entitled to have anyone else respect your preferences (any more than they're entitled to have you date them in spite of your preferences). It's important to be self aware, if nothing else, so that if your preferences are caused by internalized prejudice you can at least avoid dumping that crap on other people.