I agree Puppy!! Wanna know what's worse? Being in an SSM and your spouse 'giving' that part away freely to someone else. That is what kills my friend, that she sought comfort elsewhere after 'depriving' them for years. I also think its a bit harder on a H when a W cheats. I haven't been able to figure out why, but I think its true.
H and I were not in a SSM when his A happened. Far from it. We were actually actively trying for #3. We were in a 'friends with benefits' mode, I believe. But he quit giving himself emotionally to me, and was there for OW in ways that he has never been for me. That bites too.