Peter C:
... I've always assumed it was a strong marriage and a trusting relationship...
I believe the same. Although you see a lot of caps and writing suggesting that cuckolding is all about "wimpy men" and selfish Cuckoldresses, it's actually the opposite. For this lifestyle to be successful you need a solid marriage, a strong bond and respect for one another's needs, concerns, etc.
There was a brief period years ago when I had a job change, financial pressure, etc. Ironically, at that same time, she was seeing a guy she was crazy about. Although I'm normally very confident in our relationship, due to those other circumstances I found myself feeling very uneasy... even jealous. So you have to be on rock solid ground to tolerate your beloved spreading her legs for another guy!
cuckold - Pronunciation - kuk-uhld - noun 1) The husband of an unfaithful wife. 2) A husband whose wife has sex with others. NOTE - It does NOT say: Sexually confused, submissive, humiliated, sissified, crossdresser, cocksucker, or piss drinker