Sounds like a good strategy. You may want to take into account from the beginning that, since Frank is an "available guy," an affair between him and your wife could lead to at-first-negligible, but later steadily-growing-thoughts-in-his-mind, of prying your wife loose from you and "running off" with her (as happened, more-than-likely, with Don). You and Sue will have the best insight re. how to do it, I'm sure, but it strikes me as a good idea to make clear to Frank that, when (or perhaps before) your wife fucks him for the first time, you hope he'll remember that she is your wife and he is (or will be) her lover... not "the secret love of her life" who's standing in line to become her next husband.
Custer, you have a practical mind and the warning to Frank is valid and a good idea; however the game Sue & Stb are playing is a high risk game, fraught with danger of which they are both fully aware. Sue has "fallen" for her lovers twice. She may do so again. Frank may be a gentleman but when the chips are down and love calls with a hard dick and a wet pussy, rules tend to go by the wayside.