How about you write up a formal agreement with W about your FWB plan. You've said in the past she's OK with it. Get it in writing so that you are both very clear on the terms/boundaries/expectations. Heck, get it notarized so that she can't sue you for infidelity.
What is the harm? After all, neither of you wants a D, and both of you are OK with what it takes for you to satisfy your sexual needs outside M. Right?
