A black gay friend who has had sex with more black men and seen more black men naked and in various states of arousal than I ever will, told me how in his experience of having a super large number of male sex partners of all races but generally preferring black/mixed race men for partner/boyfriend, and watching gay and amateur solo masturbation porn that has black men in it, that 'most black men are just average and do not have large cocks.'
I mean, that data is skewed by the fact that homosexuals on average have smaller penises. Almost every gay man I've known had a below average sized penis. I can imagine this same thing carries over to black homosexuals too. As to why this is a thing who knows.