I think it's such a shame (and a waste) to feel like this, when it is possible to have it all. Butterballs asks what is the best model? Assuming we're referring to monogamous relationships with children (those where both parties have agreed to be monogamous) I'd say the best model is one where there is compatibility in all the areas Butterball cites as being important in a marriage and love and sex. Monogamy does not have to be monotonous at all.
I agree that too many people ditch their marriages for the sake of lust, but I cannot see that "discreet affairs" are the answer either. "What you don't know doesn't hurt you" is I'm afraid, a complete myth. Men and women are often so rubbish at compartmentalising when they have an affair that the primary partner does hurt very much indeed - it's just that they don't know why they are feeling sad, unhappy and unsettled.
I also fundamentally disagree that raising children in a marriage where the intimacy has gone between the parents, is healthy or provides them with a stable environment. Children really notice this too, especially when they are in their teenage years and see the differences in the way their friends' parents are with one another - my H remembers feeling profoundly sad that his best friend's parents were so loving to one another and his were not. It is a very powerful memory for him. We were discussing this recently, because a couple of our kids' friends have made similar comments at the difference in their own parents' marriage - my daughter was told that she was "so lucky" to have parents who are still in love.
People often think that as long as they are not having rows and appear to co-exist peacefully, this is enough of a good example to children. It really isn't enough though.
I am not saying that divorce is any better though (and can quite see that it is often worse for all parties) but there is a "third way". Even marriages that have lacked intimacy for years can be rekindled - divorcing or getting sex from elsewhere are not the only options.
Trundling along in a marriage that works well as a business arrangement, but which lacks passion, sounds like a kind of hell to me, as is living in a marriage where the only barrier to being unfaithful is because "I couldn't be arsed with it." If both partners are actually feeling like that, it is the perfect breeding ground for one or both of you to have an affair - and however much you might say now that if that happened, you would both be sensible and see it for what it was - a form of escapism and fulfilling an unmet need, it is impossible to predict how any of us would react in those circumstances.
At the extreme end, you or your H might fall in love with the affair partner - and want to leave the marriage; you might hurt the affair partner deeply because you cannot control how deep their feelings will go. As soon as you cross the line with someone else, there is always a risk that one or both of the parties will develop strong feelings.
Saying you would "turn a blind eye" if your partner had a discreet affair ignores two painful possibilities. First, you might not get the choice - he might fall in love and want to leave you anyway and secondly, however much you think you will be able to live with it, it is rather different when it actually happens.
I read posts on here all the time from people convincing themselves that their passionless marriages are just fine and that they are being terribly grown-up and sanguine about not rocking the boat - and either not needing sex, passion and intimacy, or thinking it's acceptable to get those needs met outside the marriage, as long as it's "discreet". I would however contend that this is actually more naive - naive to think that people won't get hurt, naive to think that all parties are immune from falling in love, naive to think that this is best for children and naive to think that a marriage is really going to last when one or both of the parties is having such fundamental needs unmet.
Consequently - and in relation to the OP's question - I have no idea how anyone can continue in a marriage where the sex and intimacy has been contracted out to someone else - no amount of financial stability and status would be enough to bring my kids up in such an arid environment and with such a crap example of a marriage.