Why are the women of black protagonists in American films always black and almost never white?

When I watched a series of Star Trek Deep Space Nine at Netflix yesterday, I was struck again when the dark-skinned Captain fell in love with a phenomenon that was - by coincidence - black. Funny, where the series is in the 24th Century and there are all sorts of Ferengis and shape-shifting. But a deviation from this scheme is not there either.

For example, the women of Will Smith are always black.

Films with black protagonists are almost entirely State-funded (e.g., Prince of Zamunda, Black Panther). Also, you do not like pairs of different main colors in the USA - especially not on the screen.

Films with Black Actors are almost entirely State-funded

By the insanely generous American film promotion?

That's because many Americans and other countries still have a problem with two "different" coming together. At that time it was also a scandal in Star Trek as a cap. Kirk & Uhura kissed each other. That was completely incomprehensible for the time and "terrible". There are movies / series in which there are mixed couples. Are not many but there are some. Eg. Big Little Lies, Guess Who - You Can't Get My Daughter! Lakeview Terrace, Romeo must die.