You have to realize that women are raised from birth to act different socially in our culture. Many men (of all races) find it hard to accept a woman who is sexually uninhibited and as a result, many women are reluctant to be sexually uninhibited. If being a wanton cock-hungry slut was the norm, your wife probably would be willing to spread her legs for a BBC in a heart beat. But like all women, she has been told from birth that "good girls" don't do that kind of stuff. As a result, she has spent her entire life suppressing her most carnal desires.
Women have orgasms 10 times stronger than men (this has been scientifically proven, look up information about orgasms and theta waves) and they also have sexual desires just as strong as men (again, psychologists have confirmed this - look it up if you doubt me). So the biology is all there to make any woman be a wanton slut, she just has to overcome her fear of the possible social ramifications (i.e., what if her co-workers found out, or her parents, what if *you* think of her differently).
My advice is to keep working at it, keep reassuring her, and be supportive. If she has been receptive to BBC porn and a BBC dildo she is already open her mind at least somewhat to the possibility. Good luck.