Pray, pray, and pray. St Michael the Archangel
Firmly say to him: “That is a disgusting desire, and I will not entertain the idea for a second. Please do not suggest it ever again.”
If he continues to pressure, I would guess it is time to start asking him questions like: Do you understand what marriage is? Do you realize that you are asking me to commit adultery? Do you realize that you are commiting adultery yourself by desiring that I have sexual relations outside of our marriage?
If you decide to file for divorce and seek an annulment, I don’t see how this is mortal sin. I am seeking an annulment currently, and I was told that the process cannot begin until a civil divorce is final. It makes no sense to me that divorce is mortal sin if I have to obtain a divorce in order to even have the Tribunal review my marriage. The mortal sin is if you (a) divorce, (b) the Tribunal reviews and upholds the validity of your marriage, and © you ignore the Tribunal’s ruling by marrying another man or seeking a relationship with another man for anything beyond a friendship.
May God bless you. I will remember you in prayer.