I don't know where it fits in on a DB scale, but what's wrong with speaking to the OMW? You're both in the same situation and can support each other, tough noogies if the cheating spouses don't like it. They're making choices -- which they're free do to -- and these are the consequences. Surprise!
I watched a movie last night called The Other Woman, in which a man's girlfriend discovered he was married and she and the W ended up becoming bosom buddies and getting through the situation by leaning on each other. You get your support where you can find it, in my opinion -- even if it comes from sources you didn't count on.
BF:40 M:33 SD: 12 T: 8, never married, no kids together BD: 8/4, "I'm just done", "...too tired and burnt to try". PA confirmed 8/5 "It happened, but it's been over for almost a year".
