Celtic, in answer to your question: "Would you expect your boyfriend/husband to be faithful to you?" Not necessarily, especially if he met someone he was intensely attracted to. I think that is unlikely but not impossible.
"Divorce is a civil right. It's there to get people out of marriages that have gone horribly wrong such as domestic violence, abuse of the children and incompatibility."
Celtic, are you suggesting that everyone who gets divorced these days is doing it for those reasons? I would strongly disagree, as I have pointed out with my examples of several couples I know. I think divorce has almost become a lifestyle decision. Again, I am not necessarily saying this is right or wrong, but it has become so commonplace that it is now more socially acceptable than infidelity, which in some ways is slightly odd, especially when you consider the fall out from divorce.
The fact is that the spark does tend to go a bit once a relationship has matured, and I imagine that at this stage it is not uncommon for someone to start fancying someone else and either start an affair or leave the existing relationship and then start a new one. I know quite a few people (men especially) who have two or three long term relatonships or marriages where this has been the case. There was nothing especially wrong with the previous partners, but they got bored and wanted the excitement of a new sexual relationship.
WhenwillI: "The reason why infidelity attracts so much bitterness and approbation is because of the deceit and lying involved, unlike when a marriage has hit the buffers because of boredom." Yes, I don't deny that. But there is also a huge amount of pain involved when when partner decides to exit the relationship in the manner above, eg: because they want the excitment of a new sexual relationship and are bored with the old one.
In the past, there would have been more of a social obligation to stick in the marriage, with, not uncommonly, another relationship on the side. This would have catered for the the (man's especially) desire for a stable family home life AND sexual excitment on the side. No doubt I will be flamed for this, but just stating what used to happen, and presumably still does to an extent.