My wife had an emotional affair about 10 years ago (I'm still not 100% sure it wasn't physical, but it was so brief that I doubt it ever amounted to much). It was amazing how quickly she drifted away and I lost her. She even admitted to me that she wanted to spend more time with one of her best friend's husbands in a tearful admission one night out to dinner. Of course, I said No, but she was already gone. It was like she was on a different planet and there wasn't really anything I could do. But, we did get into therapy quickly and it took a lot of work and me blowing up the affair to the wife of the other guy (we were all good friends). Anyway, I didn't like snooping on my wife but it was necessary for a few weeks to see what she was up to (she started setting up new gmail accounts and possibly had a burner phone). Don't ever trust your spouse 100%. I'm not talking about setting a surveillance state, but if you think they are getting too close to someone (opposite or same sex), you need to start asking questions and verify. Eventually we figured it out and have been mostly happily married for the past 10 years but it was bad for a while.