You convince yourself you had no choice and that had you stayed youd be sacrificing your happiness. Then you do what you can to not face it - another person (all the new chemicals and relationship excitement) is the perfect distraction.
Until your on your own you don't have to think about it.
That and some people don't feel guilt
Both mid 30s, 2 young kids BD 7sep14 XW moved on long ago, now living with OM1 D paperwork in progress