Anal "interracial" was kind of a hype years ago. Especially, if the woman involved was another man`s wife or girlfriend. i see it less common nowadays judging from the decreased numbers of discussions and threads about the subject on lifestyle sites and forums.
i guess one of the reasons was that anal sex is often considered a not-so-common sexual practice especially between strangers and in a ONS situation, making it something special, almost a taboo, adding a sort of extra kinkiness to the experience. Another reason might be that anal sex comes closest to penile-vaginal penetration AKA intercourse.
Many comments have been made by couples in the cuckold and/or hotwife lifestyles about giving a "bull" exclusive permission for anal sex with the wife or girlfriend. In many cases it is considered a gift for the additional player in return for his providing the wife/girlfriend and couple with that special excitement and pleasure only possible in those sorts of lifestyles.
Talking about arrangements that include "agreed exclusiveness" in terms of a sexual practice, my perception is that there has been a shift from anal sex toward intercourse. i read more often today that couples say it is their choice for the wife`s pussy to be "black dick exclusive" rather than for her "backdoor". What`s your perception, everyone?