I have to agree with Peak, STB.
Sue has hinted over the past several years that you can come back to her anytime, by telling her you want to end the beta-desires. Sue has just about reached the end of her rope. With Robert, Sue had a very serious affair going, but never let you see or know who Robert was, or how much sex with him affected her. He was apparently very good at it, and had a large cock and knew how to use it. Sue ended it, without alot of emotion left, and Robert moved on to a permanent and available lady friend.
Sue found Paul and has tried several tacts with you. She has threatened to have more sex with her lover, she has threatened to cut you off entirely, she has threatened to give you very few and far between "mercy fucks" and a year ago caused quite a rift after the ski trip by ignoring you to go have sex with Paul. Then she changed after your near meltdown, to try and include you more. That seems to have been to try and let you know, and witness, all that was going on, and possibly bring out some jealousy, or basic need. However, you seem to have interpreted it as Sue wanted to share her relationship with Paul, with you.
Sue doesn't appear to want that. She married an alpha male, and now that she has experienced many different men, and has the confidence and maturity to know what she likes and doesn't like about the men and sex, she is about to give you an ultimatum. Sue seems to want you to be her alpha-male: an equal partner in this marriage. You have that choice. If you take back that alpha male role, she will let you have sex with her bare, and then you two may negotiate if she continues with a lover, although she has made it known that she wants one for at least the next several years.
However, if you want to remain the beta, and continue to "deny" yourself bare sex with Sue by using a condom, know that Sue has told you condoms don't do much for her, as she may orgasm mildly, but to really have a good orgasm, she has to feel you bare, and she has to feel you come inside her bare pussy. If you choose this path, Sue has the signal that you truly want her to look to Paul for all of her sexual needs. Just remember that sexual needs are more than just sexual intercourse. Sue will mature those emotions she and Paul have already started to develop. Sue will feel more and more drawn to Paul for emotional support, until you will become just a roommate, and Sue will go to Paul for the significant issues that require soothing, cuddling, and lead over and over to sexual release and satisfaction.
Sue will then have no need, or desire, for sex with you and condoms, and it will truly diminish until there won't even be an offer for a "mercy fuck", as sex with Sue will be irrelevant. The closeness you talk about with Sue comes to us by your thread, and maybe "rose-colored glasses" as you two mature. Sue appears to be a mature lady to still desires alot of sex. You must decide whether you will be the one to supply that love and sex, or step back and reliquish that role to Paul.
I noted that Sue and Paul have talked, and though Paul was reluctant he picked up the ball this past weekend, and Sue crossed over the line to basically humiliate you this time. Sue has never done that before, and has been very supportive of you, and refused to humiliate you. This weekend you felt like a "puppy" because Sue and Paul took it to the next level, excluding you from any and all activities of a dating or sexual nature, and even using you as a spotter so they could have public sex. It will only get worse if you want to remain the beta.