So affairs are 'wrong', but who would want to live in a world where there are no affairs?
We excessively police and punish our partners, our friends' partners and our parents, but often hypocritically, when it comes to ourselves and our friends cheating will generally be forgiven.
I am not even advocating abolishing the hypocricy. Without it affairs probably wouldn't be exciting and equally, faithful relationships wouldn't be as meaningful.
What is more, having broken up a long term relationship with children (with objectively plenty of cause), and having not had an affair what I can say is that for a woman this can be even worse, as it leaves them without something to be mad and and just feeling unvalued. (It probably goes both ways for both genders, but in my view it is something that is clearly not symmetrical in Male-Female relationships).
However what I do think is wrong are ill feelings to the point of death threats and hatred (which I experienced) even if there is infidelity. In this situation can someone really claim to have loved someone in the first place if they value their life outside of the relationship so little. When this is the basis of someones feelings then the relationship/marriage is a kind of tyranny in my opinion.