A few different things are going on with respect to race - and racism - in porn.
First of all, the adult industry is inherently and repulsively racist. I always call it the last bastian of blatant, old school racism in western culture. It's disgusting, period.
And because of the way interracial relationships are often depicted in porn, some girls give their agents a "no interracial" blanket policy. This is so they're not booked for scenes where both races are depicted as vulgar stereotypes to be made fun of, and where the sex is orchestrated in the most demeaning way possible.
There are several girls with a "no interracial" policy who have made it clear to me that they'll shoot interracial scenes for my studio anytime. Zoe Voss is one of those girls. She just wants to make sure agents aren't booking her for scenes she doesn't feel good about being in. And yes, she has already done an interracial scene for me, with Mickey Mod.
That issue aside, there are many white performers who won't work with black men or women in ANY type of production. Part of this stems from the "conventional wisdom" perpetrated by white men in porn that "If you do interracial you'll never get signed to a contract" or "you'll lose fans" or "you'll stop getting work with the best companies." None of that is true, but many girls still buy into it, despite overwhelming evidence to the contrary.
But of course some performers refuse to perform interracial strictly for personal reasons. "I'm just not attracted," they explain (suggesting that they're "always" attracted to their white costars -- highly unlikely!!!)
As with all racism, I think it's just a product of ignorance and fear. And I've seen it on both sides of the camera. For instance, many white girl/girl fans openly expressed a preference for white, girl next door types to be cast. As a result, for years you really didn't see any black women cast in g/g scenes. But early in my g/g career I started casting Nyomi Banxxx, Ms. Platinum and Sincere Lemmore in multiple scenes, and once the fans "got to know" these women, they were requesting them like crazy.
And it's the same thing with many white performers who "get to know" a black performer and suddenly say "I don't do interracial but I want to do a scene with (Mr. Marcus)" or whomever.
When you get to know someone you stop seeing their color as a defining characteristic. And that's what we should be working on in porn, on both sides of the camera.