Oftentimes people will ask the cheater: "If you're not getting what you want or were that unhappy why didn't you just end the relationship?
Generally speaking cheaters aren't looking to "replace" one relationship with another. They're looking "compliment" what they already have.
The primary goal of most cheaters is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side. These so call "needs" vary from person to person.
They may be physical, emotional, ego driven, or for excitement.
If for example a married man with three children finds himself with a wife who has lost interest in sex. They've talked about it and tried other things but she simply has no desire. From his point of view he has three options.
1. Accept the fact he is done having sex for life.
2. Run down to the courthouse to file for divorce, move out of his house into a one bedroom apartment, pay child support and possibly alimony, become a weekend dad, and divide up family and friends as they choose sides.
3. Find a woman who is attracted to him and enjoys having sex.
Many go with #3!
A few years ago the following question was on HP.
"Is sexual incompatibility a valid reason for ending a marriage?"
Over 90% of the responses said "no". That's exactly how cheaters feel too.
I imagine some folks feel they cheat in order to (stay or tolerate) unhappy marriages. Whatever issues they're having doesn't justify divorce to them. I believe there are 3 basic types of cheaters:
1. The Incessant Cheater
This type of cheater most likely has never been faithful in any long-term relationship. They get bored easily.
Their motto would be: "Variety is the spice of life!" Whenever they attempt to be monogamous they find themselves behaving like an overweight person trying out a strict diet. Most likely it's not a matter of (if) but (when) they will cheat.
2. The Unbelievable Opportunity Cheater (Caved in to temptation)
This person wasn't proactively looking to cheat but an unbelievable opportunity arose be with a "fantasy" or secret crush who hit on them. Oftentimes due to guilt this cheater may confess at some point.
3. The Discontented Cheater (He/she blames YOU!)
Something (you did or stopped doing) led them to venture outside of the relationship. Someone else put a smile on their face and made them feel "special" again.
Cheaters never believe they'll get caught so it's a better option than divorce in their eyes.