First up I wonder why you think you have outgrown one another?
I promise you these arrangements can work, and you won't believe this right now, but it can make your marriage stronger.
I see you are in a period of mourning over the loss of the monogamous dream.
I actually did ask someone out last week, and that felt good - though she wasn't interested in a married man. It was good for me to DO something instead of just worrying about what M does.I hope he gets into the planning phase. He needs something to look forward to.