Married men cheat for the same reasons married women do.
More often than not this is a "rhetorical question" in the sense that most people really don't care about (why) someone cheats!
Nothing one says "justifies it" in our mind!
The goal of a cheater is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side.
Essentially they're looking to "compliment" their existing relationship.
Rarely do cheaters seek to (replace) one relationship with another!
If they were looking to replace their spouse divorce was always available to them.
Cheating is about breaking the rules, secrecy, and deception. Someone wants (more) than what is being offered to them and they feel entitled to it. There are three basic cheaters in my opinion
1. The Incessant Cheater
This person has never been faithful in any long-term relationship! They get bored very easily and is always on the look out for a thrill that comes with being with someone (new). Their view of monogamy is equivalent to choosing to go on a very strict diet. It's not a matter of (if) they will cheat but (when). Their motto: "Variety is the spice of life!"
2. The Unbelievable Opportunity Cheater
This person isn't "proactively looking" to cheat however under the so called right circumstances such as someone attractive (hits on them) or a secret crush makes it known there is a shot to go from fantasy to reality they might cave in to temptation. Sometimes this type of cheater confesses weeks, months, or years later in order to absolve the guilt they carry around.
3. The Discontented Cheater
This person blames you!
If you hadn't stopped or started doing whatever they would have never stepped outside of the relationship. Essentially they feel you stopped making them feel "special", took them for granted, neglected them in some way be it emotionally or physically.
(After talking with no changes in sight.)
Their options: 1. Accept things as they are. 2. Divorce. 3. Cheat.
Having said that their "unhappiness" hasn't risen to the point where they want to run down to the courthouse to file for divorce, move out of their home into an apartment, pay child support/alimony, lower their living standard, become a weekend dad, divide up family & friends.
Since cheaters don't expect to get caught many go with option 3.
It takes more courage to walk away from an unhappy marriage than it does to cheat.
Cheating is a cowardly act to attempt to get (everything) one wants!