[ QUOTE] ...slowly realising he cheated on his family for some fun and ended up within a few months in exactly the same position he was in before, married with little kids
The stress of it all and having no time to breathe and settle as a couple, just excitement and drama of an affair then a couple months of shacking up then a baby on the way.
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This is why people have affairs, it's the excitement and attention. They want a break / want out from what they see as the monotony of everyday family life. They love the attention and feeling like they're irresistible, one on the go and one at home.
You are so right, they absolutely do not leave to replicate what they've already got, it's the excitement they wanted, the ego boost. From experience, they see the one left holding the babies just getting on with life, and start some sort of competition to prove that they are just as happy. Or they realise how much they miss the ones they walked out on and replicate the family to fill the void.