I still have two teens at home. I think my plan would be:
1. Kids, you will treat this man with the respect due to any adult member of the family.
2. Kids, you will treat this man with charity due to his being the apple of your mother's eye.
3. Kids, you will do nothing that will create disharmony between husband and wife. (This is easy to explain to a teen, much more difficult when the kids are younger.)
4. Husband, you will express your opinions in front of the children calmly. You will not act like a child when others -- including myself -- disagree.
5. Husband, you will consult with me privately when you think the kid is out of line or that I am permitting/making a bad decision.
6. Husband, I promise that I will listen carefully to your advice, but you must understand that my relationship with the kids is longer than yours. I know them better than you do, though I appreciate that your "third party" point of view can bring valuable insights.
7. Kids and husband, remember that mom is performing a balancing act between raising kids and caring for a lover. Don't make me choose between you -- because you won't like the outcome.
Thanks Marge, this is brilliant.