6 months ago I would have said this is all just delusional fantasy. However, since my girl and I began having BBC threesomes I can attest ... it's very real. Some have said you should be careful because you can't go back. That's true enough, but as you've correctly stated, she's all ready seen the pictures and read the comments. I can assure you, she's thinking about it ... a LOT. The deeper truth is, most likely, it's inevitable now ... with your approval or without.
What loveBLACKmen said though harsh is true as well. The black guys we had threesomes with have been quite dominate in the sack and white women (at least mine) respond to that. From my perspective she's become a total slut for BBC. Her well hung lovers are openly contemptuous of me now and with encouragement from them she has become so too. We still sleep together but she gets her satisfactions elsewhere now. If she sees me naked in the shower, the most I get these days is a condescending smirk.
Once you go down that path ... it's out of your control ... period.