That is exactly the point. There's nothing wrong with preserving culture. The idea that "I shouldn't be dating someone because that person should preserve his/her culture" is where the sentiment crosses the line. It makes preserving the culture an imperative, rather than a choice that each individual freely makes.

Of course, what makes things even sillier is that she starts off by saying "he identifies himself as African-American", which to me says he's not 100% black to begin with.