I agree that its always happened and always will.
Re problems within the marriage=affair, I think there are lots of relationship situations out there where the two parties are on significantly different pages and yet plunge into marriage/kids regardless, that has the potential to lead to marital problems which in many cases one partner will use as justification to have an affair.
If your spouse has an affair IMO there is a problem within your marriage, I don't mean the problem is your doing, but there is something not balanced if your spouse can have an affair.
My EXDP cheated, when caught despite all the 'she's awful' guff he fed the OW about me, he was begging me not to leave him. I left him anyway.
Best thing I ever did as our relationship despite its longevity was not good looking back on it.
Had I had a crystal ball many years ago I would have seen that he wasn't right for me or equipped to handle a serious relationship, I suppose in my naivety I pushed it forward regardless.
I think some relationships are doomed from the off and affairs can be part of the ongoing script as sometimes people see them as an escape.
I struggle to see how anyone in a 'good' marriage/relationship can have an affair, there must be something wrong or it wouldn't happen!
I think some people (mainly women) are offended at this suggestion as they take the 'something wrong' part to mean something wrong about them or in their behavior. I don't see it like that, I think its the issue of the person having the affair not their innocent spouse.
My EX is still with his OW, he is also still a philanderer! The common denominator is him its his problem/issue.