The problem with the talking to your partner and agreeing to open up the marriage or split theory, is that very few partners would agree to that, which would lead to a split. And lots of people who have affairs love their spouse very much and their family and life, and they don't want to split.
By the logic of the blanket condemners on here, so that the adults can say they are being "principled", the marriage has to end. The fact that this may cause untold distress to the children, the children only seeing one of their parents at weekends, the loss of thr family home, possible poverty - all that is ok as long as one adult isn't "immoral" by having an affair that they probably don't want, if their wife would just have sex occasionally.
Why is putting children through all of that more morally acceptable than an adult having a discreet affair? In one scenario the only people affected are the adults, and perhaps the children if the affair is exposed, in the other scenario, the children are definitely affected.
As for IMO some people can love more than one person at once in slightly different ways - it's to be bloody hope so otherwise the kids of parents with more than one child are screwed!
We all love different people in our lives in different ways. Lots of people feel able to love more than one partner intimately. It's too cut and dried to say "if he loves her he can't love me". Because very often it isn't true.
I get that that might be a terrifying concept for lots of people, but it doesn't make it less true. People don't always behave well to people they love. If they did, teenagers would never be shits to their parents.
The myth that people can't love some one that they cheat on is so corrosive. I have seen it counselling people countless times. It's a myth we tell ourselves "he loves me so he would never cheat". It gives us an illusion of control. But an illusion is all it is.