I guess it all depends on what your own core boundaries and beliefs are, and whether or not you're willing to live in an open marriage. Because that's clearly what your wife has flat-out-to-your-face proposed to you here.
She's testing you, and it would seem like she's been slowly turning up the heat over the years to see how much of this you will tolerate. I can only tell you what I would do.
How would you counsel your son, if he came to you as his dad and said he was in this exact same predicament, 30 years from now?
