That may be how it came to be for some. I wouldn’t say most. I know for sure that experiences in formative stages —particularly involving instances of female infidelity that come to be eroticized by the male — figure for plenty of men. I think we’re in an age of increased empowerment for women, when such cheating transgressions are far more likely to be tolerated and accepted by a male partner, not nearly as likely to be the automatic relationship killers they once were. Plenty of couples work through that kind of thing, and in doing so some must unwittingly sow the seeds for a dynamic that is fertile ground for cuckoldry or some form of sharing. Is that sick? If they understand each other and both enjoy it? If it’s not abusive or harmful. I really don’t think so.
I do however, think it can be harmful and detrimental to be mired in thoughts of any kind that erode or isolate you from real relationships and undermine connection. Can’t imagine there’s much positive or productive in being alone in it if you have a significant other.