Xtra47:
I'd bet it's the wife more often than you think.
I think women are quite clever at engineering the situation so the husband ends up believing it was his idea for the wife to start having sex with other men. I never had these tendencies with my first wife nor the woman I lived with after that, but they started to surface with my 2nd wife Debbie.
As I worked shifts - one week early, one week late - Debbie gradually developed a social life of her own. This might involve dr1nking at our works' social club, out with female friends or out at bingo or playing darts for a ladies team around local pubs and clubs. Sometimes, after a few dr1nks, she'd come home and tell me in bed if she'd been chatted up or kissed and cuddled. Debbie clearly realised that these little confessions of hers aroused me as we made love and eventually I found myself fantasising about her going further.
With the help of some role-play in the bedroom with a life-like toy, we were soon pretending I was watching her having sex with another man. I liked seeing a different "cock" going in and out of her and told her how naughty she was doing it with him right there in front of her husband. "You love it!", she'd murmur as she climaxed more noisily than usual.
From there it was a short step to her doing it for real with my knowledge and full approval and although we set out a couple of ground rules to start with, for her to use condoms and not go with anyone I knew, these were soon discarded with no complaints from me when she fucked a workmate of mine bareback. He'd been after her pussy for years and couldn't believe his luck when she finally gave in and let him have what he wanted.
Her pussy, her rules and I'm sure from me not being angry about her being chatted up or fondled by guys when she went out, she was able to slowly but surely push my boundaries which enabled her to easily get me to agree to her fucking whoever she liked, whenever she liked.
Peter C