HI there, sorry for the delay. I hear your concerns and have some thoughts about it. First, you messed up big time, the damage is done, but it can be repaired. Second, your husband has not forgiven you. He holds it over your head and he wants retribution. This is not forgiveness and it is not helping the both of you to repair the relationship. Third, involving other people into your sacred marriage in a sexual manner will not solve the problem. It will cause you problems like it has continued to cause him problems. Fourth, honesty is key throughout all of this. Finally, I think the two of you talking to a mutual third party and hashing out some of these feelings would be most beneficial. There can be no moving on and healing until this matter gets resolved. I hope this helps.
I like how you say that the two of you are sexually open minded, yet once you brought a third party in then these problems arose. My thoughts are that if you do it, he will feel like he got his way and since he got his way then there will be room to get his way again in the future. Either the two of you are committed to each other or not. Bringing in other people to have sex with only causes more problems, as we have seen already.