So your emotional affair turned into a physical affair. Very common. I think you should either 1) Cut all ties to K and never contact him again and never mention any of this to your husband, or 2) Come clean with your husband and, based on what he wants to do, work with him to mend the hurt you will have caused him and vow to work on your marriage. I'm not at all a fan of entering polyamory straight out of an affair. I know it occassionally works, but only after a LOT of work to mend fences and establish a lifestyle of honesty and openness.
Think about it from your husband's perspective. He's already said he wouldn't be comfortable with you seeing another man, yet you are engaged in an affair anyway, even though you know it will break his heart. You have a lot of work ahead of you, and it can be done, but try to take the moral high road on this. Don't engage in behaviors that endanger BOTH marriages.
These thoughts are my own, and based on my experiences being cheated on.