When I became involved with my first Poly boyfriend I hadn't even considered being introduced to his wife. I was making it up as I went along, so I guess I had assumed that the usual approach to such things would be DADT. I found that meeting her was freeing, in that I was clearly not a dirty little secret and therefore didn't need to worry that I was stepping on her toes.
I think I would be a lot less comfortable in the relationship if his wife and I didn't now know each other fairly well. She trusts me enough that she's asked me to look after him for a couple of days following minor surgery; for various reasons she won't be around to keep an eye on him herself.