No. Logically it makes no sense to believe that it is possible for a person to be an (ex) smoker, drug user, or drinker but if he or she has cheated they are forever stuck cheating for the rest of their lives!
Monogamy is a lifestyle (choice) not a DNA code!
"The once a cheater always a cheater" is a self-defense statement one makes to protect oneself from taking a (risk) they may regret.
Everyone is entitled to have their own "mate selection" criteria or rules, However to say someone who cheated on their mate when they were 18 is likely to cheat on their mate at 38 or 48 denies the possibility that people can learn from the past, mature over time or evolve.
Lastly there are many instances where people overcommitted due to lack of knowledge of themselves especially during their youth or they were unrealistic about their expectations.
Any teenager who believes they have found their "soul-mate" before figuring who (they) are is likely to experience heartache. One's youth is a time for learning, exploring, and discovery. Dating for them should be a FUN sociable activity and not some exercise to pretend as though they're adults.
Another common mistake is making assumptions. Other than actually having sex with someone behind their mate's back many people have (varying definitions) of what "cheating" means.
One person might consider having "secret friends" you correspond with or meet for lunch/dinner to be cheating, another might consider having a long distance relationship online with someone you've never met to be cheating. Others break it down by physical action; holding hands is cheating for some folks while others feel it's not cheating unless there is some form of sex.
Finally you have those who never discussed being in an "exclusive relationship". They just "assumed" they were and when they discover their "mate" is also dating others they brand them as being a "cheater".
Communication trumps assumption any day! Never assume you're exclusive. In order for there to be cheating a couple has to define their relationship and expectations of each other. Only then can you say there was betrayal if someone "breaks the rules".