Lets say a 40 years old guy marries a 20 years old. In 20 years, he will be 60 and she will be 40. 40 years old do not usually have the same energy as 60 years olds. Why would a 40 years old woman stay with grandpa when she could fuck men her own age? She will cheat. And this is the real reason why age gaps have a big divorce rate.