Neither gender is standing on "holy ground" when it comes to cheating. Even lesbians have cheating partners!
The goal of most cheaters is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side.
Very few people cheat with the idea of replacing one relationship with another. They're essentially looking to "complement" what they have.
Some people cheat in order to stay in or tolerate bad marriages rather than file for divorce and change their financial living standards.
Most cheaters don't expect to get caught!
Having said that I believe there are 3 basic types of cheaters.
1. The Incessant Cheater
He/she get bored very easily. They have probably never been faithful in any long-term relationship. They long for the excitement that comes with being with someone NEW. Their view of monogamy is the same as going on a very strict diet. It's not a matter of (if) but (when) they will cheat. Their motto is: "Variety is the spice of life!"
2. The Unbelievable Opportunity Cheater
This person is not "proactively" looking to cheat. They may have had a secret crush on someone or thought she/he was "hot". One day this person actually hits on them! or they make it known they're open to hooking up. This could occur on a business trip or some other outing where his/her mate isn't likely to find out. It's an opportunity to turn a fantasy into reality. This person is guilty of caving in to temptation.
It's not unheard for this type of cheater to confess weeks, months, or years later in order to absolve them self of the guilt they're carrying.
3. The Discontented Cheater
He/she blames YOU! There is something you did or (stopped doing) that caused them to be open to cheating. This could range from being emotionally distant, lack of sex, constant arguing/complaining, taking them for granted...etc Someone came along and flirted with them, complemented them, put a smile on their face and in their heart, told them how LUCKY (you) are to have them and made him/her feel "special" and "alive" again.
This gave them something to look forward to!
Both monogamy and cheating are choices made by the individual.