You could argue that running off with someone just because you fancy them or have feelings for them, ditching in the long-term relationship, throwing the whole family into emotional, financial and other turmoil is quite selfish.
Or, another way of putting it, the "in love" model of serial monogamy where, every time you want to have sex with a new person you ditch or divorce the old person and start all over again, maybe to repeat the process futher down the line, is extremely distructive to family life and children.
I know of several men who have done this - there are two or three ex wives or partners, god knows how many half and step children. It's all incredibly messy and complicated and they are no happier than before - and considerably less well off. Some of the children are emotionally damaged by messy break-ups and there is lots of bitterness.
Yet, the monogamy die hards would argue that this is being "honest" and "authentic". Nevertheless, it is still breaking marriage vows, over and over again, if that is the issue.
The old style marriage model where people stuck together but, sometimes, had relationships outside the primary one, was and is, in my opinion, better for bringing up children IF the couple fundamentally get on.
It's not an especially radical view, I can't understand why everyone gets so agitated about it.