Solution : Let's users add their own ethnicity or select from list of other user added ethnicities.
Why not just stick with the facts:
* What color skin do you prefer [white / tan / bronze / dark ] etc
* What countries of origin are you interested in
[Africa / Jamaica / England / poland / ukraine etc etc
* What religion
It's fine if you don't know/care what your own ethnicity is, but to say that you don't understand why it's important to anyone strikes me as moral posturing, or an Asperger-level lack of empathy with other people. Some possible reasons why people may care about their own or their partner's ethnicity:
1) Knowing their susceptibility / children's susceptibility to genetic diseases.
2) Wanting their children to be raised in a certain culture
3) Wanting a partner with certain shared experiences/world view
You adjust your "tag" based on your ideal date. If you tag yourself in a narrow way on some attribute (ethnicity, profession, interests, whatever), it means you do not want to be found through searches on that attribute, except by people who use that exact term. But you DO consider that attribute important enough that you want to send information about it to people who found you through searches on other attributes.
And on the other hand, you may provide a very broad tag for "ethnicity" (e.g. "Black", "Asian", etc.). That means you want to be found by people who search using those very broad terms. And on the gripping hand, some users may choose NOT to provide a tag for their ethnicity. Their ethnicity may be reasonably obvious from their photo, but their failure to provide a tag also sends a signal --- namely that they don't want to found by the kinds of people who search based on ethnic/racial background, and would resent being set up on a date with them.
Basically, free text allows not only for signalling about one's race/ethnicity, but "meta-signalling" about how important you think race/ethnicity should be on a dating site. Satisfies a lot more use-cases than a drop-down field which requires you to fill it out.