Interesting, thought-provoking thread.
Two infidelity scenarios:
(1) XP's parents divorced in his teens. For two years, they slept in separate bedrooms, argued a lot, and his mum repeatedly asked his dad to go into counselling with her. He wouldn't. The relationship was well and truly on the skids. She started a relationship with another married father. Their affair did the final breaking-up of two already broken families. 17 years on, they are still together and happy. I don't like what happened. Nor does XP. But on a level, I understand why it happened, respect her for doing everything she could to repair her marriage/family before it happened, and their "fling" has an integrity to it in that they found their soulmates in each other and are still together - they didn't just start shagging about. Plus they handled the split incredibly sensitively for the sake of the three kids they had between them.
(2) XP, on the other hand, left last autumn after a crappy year for the bachelorhood he never had and feels is his entitlement. Before we separated, he began a drunken fling with a random woman so that "when we split, I won't be alone, because you know me, I need affection". He booked a threesome with prostitutes because "I knew then I would have to go through with the split". And all so that he could be free. He would not entertain counselling (even though he always said he would during a strained time - he hated it that his dad wouldn't) because he expressly wanted to sleep around. I have not been replaced with a new, improved soulmate: a single life of casual sex is simply more appealing to him than being in/trying to repair our relationship and family, and that hurts a lot.
Unlike his mother's infidelity, his is a kind I cannot, on any level, understand nor tolerate. We were in a sexually exclusive relationship - he wanted to get married! So never mind "poor him" being incapable of monogamy - and for me, it is the deceit, his utter lack of respect or value for me and DS and our family, his irresponsibility and selfishness, and his lack of remorse and grief, that I cannot abide and which hurt so much. Pit of despair he has indeed created!
I agree with morningpaper and madamez in that monogamy is not for everyone; that polyamory or an extended shagging-around singledom are perfectly acceptable so long as all parties are consenting and content. I think choosing the latter option after creating a family is not on, frankly - and, if someone truly has an epiphany (which XP doesn't seem to have had) that they are non-monogomous once they already have a "committed" relationship and kids, there are ways to conduct oneself which are at least half-respectful to your partner and children.
The price of my XP's fleeting moments of happiness, as he hops from bed to bed, are a broken family, a confused and sad DS who doesn't understand why his daddy doesn't want to live with him anymore, an ex-partner (me) whose trust and self-esteem are pretty much obliterated, ashamed parents who feel they have "lost" their son, and a bewildered extended family and circle of friends who have lost respect for XP. How the hell is it worth it?!