SGB
I think it is rather naive to keep suggesting that sex is just another activity like going to the cinema or for a walk.
Maybe not for you, but for other people, sex and emotions are linked in a way that a walk or a cinema trip is not.
I think you have a problem with understanding jealousy- it appears to be an emotion which you cannot empathise with.
I'd say that if you lack the jealousy gene, you are never ever going to get why some people cannot give their partner permission to be unfaithful, even if sex is not working for them as a couple.
it's not logical- but emotions aren't by their very nature!
Having said all of that, I do agree that if a partner goes off sex, withholds it as a form of power, has a medical problem which they do not take ownership of, or whatever, then they have to appreciate it will impact on their relationship- and the other partner may get sex elsewhere. But it's not really about sex is it? it's about not taking responsibility for the relationship and sex is the aspect which suffers.
There is a line of thought which says yes- sex in a solid relationship is just that - a fun activity like going for a walk etc ., until it goes wrong- then it assumes an importance which a walk doesn't.