@jsku
"When young people tend to look at things in a black and white way."
Age doesn't equate with knowledge and a free pass to hurt.
Op, I assume you are not just gaining sex from this extra marital affair, you are gaining friendship, support, an ego boost and a secret life which is making you feel you currently have the upper hand in your marriage.
In short you are taking your husband for a fool and a mug, you are also doing this to his unsuspecting partner, who incidentally you know nothing about except the information that has come from him.
Twenty years is along time to not have sex but there are other ways intimacy can be achieved, many older couples navigate this and do have rewarding love lives, but it takes communication and the desire for both parties 'to give'.
Your resolution skills show me that you were probably not entirely 'giving' within the relationship yourself, it takes two to keep a relationship alive, loving and respectful, your h needed a careful approach to having a quite devastating condition afflict him, why in all these years have you both not communicated.
Any it's pointless now, your primary relationship is ruined and quite frankly I wouldn't be surprised if he already knows that your head has been turned, what a dark thought that he maybe in pain at this very moment whilst you wax lyrically about someone who is throwing his attention at you.
You seem to think being open and honest would ruin your family set up, are you not interested in your h's happiness.
Do you think your friendshp with this man would last if you took the sexual aspect off the table ? Sex is not the only part of a marriage, especially a long marriage, maybe your h would like a friend just like you have.
Maybe if you were to come clean your h could find a friend maybe his wife ?, they could support one another, through a shared pain they could really connect, hell it could even give him the confidence to be intimate with someone or seek solutions for it.
This could be your chance to give him some happiness in the marriage, imagine them both connecting being able to go out with one another, caring for one another without sex, interesting conversations, dinners, walks, affection and maybe even love, an emotional affair born out of a shared pain, how lovely that would be for them both, anything is possible once you open up things up to a level playing field.
Why not, take a chance, give them both the right to be able to have intimacy with someone else without the need for sexual health checks before because if I were them I would want to know.
You now currently don't have any right to want what you want without consequenses, you both took away your partners choices and rights, forget about what you need think about what they need. It's only fair.
You made these decisions without consultation, it appears you are sorted, I think the least you can do is be honest and set free your h to have the chance to spend his free time being loyal to another woman/person instead of you, because your loyalty doesn't exist.
Sorry it's not a post which validates your right to hurt people just because within YOUR marriage you had a problem and I doubt this is all about the sex, it's about your right to be selfish and disloyal.
Your marriage ain't worth much to you, but I bet your h operates on a slightly deeper level, and there are similarly deeper women who would probably find him and his loyal qualities very attractive.