My experience....is that Infidelity, by itself, is almost never the integral issue.
Infidelity is a symptom of a bigger underlying problem in a marriage that isn't working.
People in happy marriages don't cheat.
Not to say there aren't marriages and couples who survive a bout of cheating...but it's a huge red flag that more is wrong than meets the eye.
Without addressing the underlying issues, the marriage becomes doomed.
Remember that 85% of people who get bariatric surgery get divorced.
If you're "happily married" and cheating...you're either a massive narcissist, or are in serious denial.