He mentioned in passing, at one point, that she wasn't happy about "our situation," presumably because what started as "just sex" has lasted for a while now, and love and friendship have developed as well. I guess that his previous extra-marital partnerships had been shorter-lived, and more compartmentalized.
The fact that he brought up something that was challenging leads me to believe that he is telling the truth. If he had said, "oh, yeah. she knows. she's fine with it..." that would be easier to just say, without substantiating it. but, the fact that he brought up the challenge... that makes me think that he is being honest.
I was traveling when he shared this with me. I immediately offered to back off - either dial back the emotional intensity, or even break off our relationship. I'm not in the marriage-wrecking business. He tried to assure me that it was going to be ok. ...this is where I have to be immensely trustful of him- that he isn't just trying to have his cake and eat it, too. I have to trust that he really is making this work for her. It's hard for me to know, since I don't have a relationship with his wife. But, he's been respectful, courteous and loving towards me... so I can only assume that he is that way with his wife. Also, if they have issues, between them, they pre-date me, and they will post-date me.