What do you do when wife wants to have an affair with a very well hung black man?

Whatever you are comfortable with. That's not something that somebody on the internet can tell you what to do. If you are under no circumstances OK with it, tell her no, she can't do it. If you are OK with it, but only if you are there watching, tell her that. If you are OK with it in general, tell her that. It's up to you and what you would/wouldn't be able to handle.