@comebacksunshines
I don't think people discussing that it's happened throughout time are trying to convince others to disregard the pain of this. In fact, I think it's the opposite - it's trying to make seen what is too often hushed behind closed doors for people to bear on their own and discussions on ways to make it less common.
That isn't what I'm getting from this thread, what I am hearing is people attempting to normalise shitty behaviour and dismissing it as the 'human condition', 'to be expected in a long term relationship' , 'decent people can cheat' , 'we're animals' etc. not much condemnation of what amounts to despicable behaviour going in in this thread.
The initial question was if it's more common than we think, and for many it is - depending on how common you think it is in the first place.
People are explaining that we can wear rose coloured spectacles and believe it is a rare occurrence, or we can look a little deeper and learn it's not particularly rare at all.
People then start to point to the abhorrence of it, the morality of it, the damage done etc, whilst others try to explain why it happens or how humanity, good and bad influences it.
People are trying to explain how it isn't something that happens to 'other people'. It can happen to folks that never expected it, or sought it. It's not condoning it, it's spelling it out for folks who think they are immune or morally superior.
Of those that say 'I would never do it' - many don't and some end up doing so.
Of those that say 'I never have' - many haven't, and some are lying.
In the course of a lifetime, there is an extremely high probability that someone close you (partner, parent, child, sibling) will have been unfaithful. They aren't evil-doers. They are normal people.