Its an excuse he uses to assuage his guilt. I wouldn't be a bit surprised if he hasn't been cheating on you since year two of your 9 year relationship. Have you ever had a gut feeling that he has already? Has he ever shown you suspect behaviour?
If you're not going to leave him for having completely different relationship boundaries from your own, then why not ask him to tell you when he's going to cheat so that you can sample some strange as well? Women are sexual creatures too after all.
As for your actual question. Don't be in denial. Of course you know that all men don't eventually cheat.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion