You're answering this like a child. The life is a lot more complicated than your black and white point of view on fidelity. When you've spent 20 years with someone, you have children, a home, cars, retirement pensions, and financial/life style/emotional dependence you may look at life differently. Like you said, infidelity is indicative of other problems; they are rarely about ***, but these problems can be fixed with desire and effort. It's like a leaky basement. When the basement floods that isn't the cause, that's the effect due to problems with your sealing and foundation. It can be fixed and you can have a dry basement. You just have to figure out where the water is getting in. Do you just sell your home when your basement floods or do you fix it, because you've invested a lot of time, love, and money in it?
Everyone has bottom lines that they won't tolerate. For most people it's abuse. Infidelity isn't as common as people would assume either. A study by Laumann et al. (1994) found that only around 1.5% of marriages experience infidelity, and around 25% of non-married relationships experience infidelity.
All he was saying is that infidelity doesn't have to mean the end of the relationship. Many people feel that way, which is why marriage counseling has a high success rate for people willing to repair their marriage. People are human and make mistakes. Sure if you're dating for a few weeks and you see your girl kissing a guy by your locker, dump her, but it gets a lot more complicated for adults.