I do not believe anyone gets married thinking they are going to get divorced or cheat on their spouse. The automatic answer to your question is no, there is not a legitimate reason to cheat on one’s spouse. However I’ve learned over the years there is a big difference between life and the hypothetical. The majority of people if they are being honest have done or said something they thought they would (never) do.
Basically there are three types of cheaters.
The (incessant cheater) is someone who flirts a lot, gets bored easily, is generally charming, fun to be around, and subscribes to the “variety is the spice of life” quote.
Most likely she or he has never had a truly monogamous relationship. They may try to settle down but will never be able to conform. Sex with (one person) for the rest of their is like going on a strict diet after living at the buffet.
The (unbelievable opportunity cheater) is someone who is not on the look out to cheat. However if a major temptation falls into their lap such as a chance to be with (someone out of their league, a person they’ve always had a crush on, a celebrity, or someone with notoriety) comes onto them or makes themselves available for one night and their spouse is unlikely to find out...etc, In their mind it’s like they won the lottery. This type of cheater is known to confess months or years later to (relieve themselves of guilt) especially if they have a wonderful loving mate.
The (discontented cheater) is someone that on some level feels “justified” to cheat.
Their mate does not satisfy them sexually, neglects them, verbally/physically abuses them, takes them for granted, is unromantic, lacks passion…etc
However the would be cheater also recognizes there are some perks for staying in the marriage. They want to hold onto all that is good while addressing their other needs on the side.
Whatever one decides to do in life there are consequences. Ideally you want to select a mate who wants what you want out of the marriage. If that is not the case then it’s possible he or she is not “the one” for you.