Marlon Wayans believes he's gotten to the root of why ex-WWE star Cameron doesn't date black men -- and let's just say size matters. 

As we previously reported, Cameron -- real name Ariane Andrew -- says she's not racist, she's just more attracted to white men. She says it's simply a personal preference. 

But Marlon says it's deeper than that ... pointing out that Cameron experiences a lot of physical pain as a wrestler -- "she don't want it in her vagina, too!" 

Stereotypes -- they're great when they're about big penises.