Frank
Steve, I hope you play it well with Frank. I think it will be no easy task, and it would be better if Sue accompanied you with an agreed dialog already rehearsed.
They would have been lovers 2 years, April 11, 2013, but longer that you and Sue have known him. Although Frank has carefully avoided becoming, 'emotionally' & 'Romantically' involved, he has provided Sue with many hours of sexual pleasure. Has she told you what, she has told Frank, other than that their sex will be less frequent?
This has been a totally different experience than it was with Brad, and with Don. Then the decision to 'break,' was obvious and timely. With Frank, you both wanted the 'friends with benefits' relationship they have had, and may continue to have, tho less frequently. But there is more to consider:
I still think he doesn't fully understand the relationship you and Sue have, that allows her to have other lovers (especially where it involves your 'denial'). He may not fully understand, even if you tell him, but if he is to know that Sue wants to explore more sex with other men, you should try to explain why.
In Frank, I think you have chosen a 'normal man'. It could be that neither you, or Sue know how to relate to a 'normal man' from a cuckold/cuckoldress perspective, but you have done it for nearly 2 years.
I think that there is nothing wrong with Frank. He responded normally to a woman (Sue) who enticed him into a sexual affair. Since you were already friends, he was understandably cautious, and when he came to know you had knowledge of it, he assured you that he would not attempt to take her from you. He has kept his promise Whether he could have 'taken' her is not the issue. That he respects your marriage and friendship is.
Although Frank don't fulfill Sue's desires now, he did fulfill her desires in the beginning. He has stepped up to most of what she has asked of him. Thats better than most marriages, Isn't it?
It's just that Frank wouldn't "push", Pull" and "dominate" her. That is what she is looking for now. That too will pass.
When it does, Frank could still be there to 'catch her'. And he may be the best one to do it! She did say that she will always have a "soft spot" for him, Didn't she?
You should not be anxious to move on to the next level. Let Sue do the exploring. You have had your turn at 'pushing the boundaries'. Now she will go even further. She will scare you even more than you have scared yourself. Then she will be back. You must be prepared to be her champion when she does!
Cheers, Harry