[K. in Cleveland]

So, what exactly is your question here? You want to know if you let her have sex with a black guy if she would leave you? You must be worried about that old saying... well, if it's something you want to try, there are plenty of ways you can go for it. It might feel safer for you if you find another couple, as single men pose a threat to the male in a relationship by not having anything to loose. Whereas a married man might have children, a home, nice car, etc, etc. to loose if he decides to take your woman. You can look online, there are plenty of sites out there for that, or find a club in your area. Or, if you'd rather it be a one time anonymous thing, then there's adult movie theaters, or you can go to a bar or dance club and pick up a guy, go to a hotel room for the night, try it out, and leave.
But the most important thing is that you ask her what she wants to do. Maybe the fantasy of it is more appealing to her than if she were actually going to. Interracial things can be very difficult for some people, depending on how they were raised. And that goes for all races, ethnicities, and religions... Talk to her, see what she wants, and if she would like to have an experience with a black man.