Both women and men cheat for the same reasons.
Every situation/relationship is different just as every person is different. None the less the person cheating has a "need" of some kind. This is the motivation for all human behavior. We tend to do what we feel we need to do in order to feel good.
Having said that (there are 3 basic types of cheaters.)
Once again the reasons are the same for both women and men.
1. The Incessant Cheater
This individual has no intention of honoring any vow or promise they make. He or she is always looking for the next thrill or something "new".
Their motto is "Variety is the spice of life."
2. The Unbelievable Opportunity Cheater
This person actually intended to be faithful and is not "looking " to betray their partner.
They give into temptation when an "unbelievable opportunity" presents itself.
An extremely attractive person who shows interest in them, a girl or guy they had a crush on in school comes to town and wants to "hang out" and last but not least a famous celebrity or politically powerful person offers what appears to be "A once in a lifetime opportunity" to have sex with someone of their stature
Their motto is in line with Oscar Wilde's famous quote.
"The only way to get rid of temptation is to yield to it"
3. The Discontented Cheater
This person believed they found their soul-mate and never dreamed they'd be considering cheating. Unlike The Incessant and The Unbelievable Opportunity Cheater, The Discontented Cheater feels as though his/her partner is forcing them to "seek other options" outside of their relationship. It's not a stretch for them to feel "justified" in seeking other ways to fill a need which is not being met. Depression often serves as motivation to take a risk.
Octavio Paz said, "A flower without a stem, is beauty waiting to die. A heart without love, is a tear waiting to cry.
I think the reason most people ask "Why?" is because they hope they can prevent something bad from happening to them or find a way to stop someone from doing harm to them. (Cheating)
In reality the only people we can completely control are ourselves! You can't stop anyone from cheating who wants to.
All you can do is look to become involved with people who place "HONESTY" at the top of their relationship acquirements.
Keep the lines of communication open and never neglect or take your mate for granted. Thats all YOU can do. Your power comes from selecting the right people to be a part of your life not changing people into what you want them to be.