I think we're on the same wavelength. I fully agree, having a couple where they are open about who they sleep with would make their own marriage better rather than sleeping around (either one, but lets keep it to her for arguments sake because of the nature of this forum).So, by your own reasoning, there's a 50/50 chance that I'll run into a trusting, caring couple (in fact, I have been with dozens). The difference between our thinking is I get off on having a guilt free sexual get together. I have no guilt, she has no guilt and the husband knows what's going on--whether he's there or not. Actually, I've had several husbands knowingly let their wife come to me for an afternoon or evening of hot, passionate sex. The lady is less tense and far more open than she would be slipping around behind her husband's back. She can't wait to tell him about it and he can't wait to hear the details (and perhaps they do their thing, clean up/fuck, after she gets home).
Many couples who end up divorcing tends to be of boredom, tired of the same cock/pussy every night (or most likely in such cases, not at all anymore), cheating happens (or at least a new love interest), the other finds out and they're headed for divorce court.
Introducing a bull into a relationship could well have fixed many marriages. If the husband accepts his own limitations (whether it's dick size, endurance, colour, whatever it is), but he meets all her other expectations, and they're both/all happy with the arrangement, it can only be a positive to a marriage.
Going behind each others back will rarely be a good idea, and of course for the bull as well. Dealing with an angry husband is probably not high on his list of things he wanted to do that day.