Kissed a manikin

Kissed a manikin: The expression to describe the phenomenon that takes place when everything is perfect in a relationship and you take it for granted and cheat on your partner and ruin the perfect relationship. The manikin is the person that you can't have.

the grass is always greener on the other sidehindsight is 2020

John and his wife had the perfect relationship, until John "kissed a manikin".