1) Roots Needed for Growth-The relationship never established roots before it became a sexual relationship. Love, trust and respect needs to be established before the relationship becomes physical. For men they need time to build up that sense of love and bonding. Sex raises the bonding hormone oxytocin but after it's over it lowers considerably unless love is established. It's basically the chemistry of emotions in the mind. Criminologists have discovered how the influence of love, sex, and romance can reform the most hardened criminals, but similarly drive a man to steal, cheat, or commit murder. On a positive note, it's the same emotions that can be the driving force in great heights of super achievement in some men. Studies show that oxytocin is linked with monogamy:
"A study, published in the journal PNAS in November 2013, examined brain scans of men who had received oxytocin or placebo via a nasal spray. The oxytocin was associated with activation of the men's reward centres in their brains, and with greater feelings of attraction to their partners versus other women in photographs. This followed a very similar study in The Journal of Neuroscience in November 2012: A hormone can help keep men faithful."
Time + Passion + Love = ?

3) Religiosity/Upbringing- a very religious man in a religion that promotes monogamy will be less apt to cheating because it's likely he's God fearing. If he grew up in a home where both parents respected each other and a high value is placed on fidelity, however tempting it might be, he will feel conflicted about breaking that norm. Our belief system stems from our culture, religion, views imposed from our upbringing and life experiences. Think about the sacredness of religious items. If you view marriage as sacred, you'll treat it differently than you would if you believed it's disposable.

4) Character Flaws- this is found in the man who will throw away the greatest woman/relationship for nothing. He will cheat on the beautiful devoted wife because he's upset, looking for an escape, to pacify negative emotions, boredom, or to compensate for something he's lacking in ego. These men are poor at problem-solving, communication, coping with stress, easily peer pressured, conflict avoiders, and usually an addiction to alcohol or something else like sex isn't that far behind. Character flaws on the female side of the relationship, is in regards to conflict resolution. Behavior/speech that disrespects, emasculates, belittles, or breaks down a male counterpart, plays a part as well. Women have to look at themselves as builders; "behind every successful man is a woman". You catch more flies with honey. Don't let him fly away because you're offering vinegar. Today our generation are characteristically flawed because nobody can stand to be in a relationship for the long haul. The minute things get hard they run. The mentality is if something is broken..."throw it away". It's one thing to work hard on something that shows no change, and knowing when enough is enough, but even then courage, ambition, determination and strength are the qualities required. The belief system, character flaws, coupled with lower religiosity, raises the likelihood of cheating.
5) Power, Charisma, Appeal- we all know some people are fierce, they're born with a power within them. They can command a room without much effort. Some strong signs in astrology create these personalities, it's just the way nature created it. I strongly believe some men just can't be monogamous unless they find someone with equal inner power as them. This power can be used negatively or positively. Used positively, you'll find these men as successful leaders, important business figures, inventors, and athletes. When we see these powerful figures fall, it's usually due to overindulgence of some kind and many times that overindulgence is another woman.
Think about all these factors. Start investing your time, energy and love in the places that need work. Life isn't perfect, people aren't perfect, but....
Time + Passion + Love =
(Anything you want to cultivate; strength, commitment, fidelity, power, growth, bonds, happiness, etc.)
Other Interesting Reads:
Why Sex Should Be Treated As A Spiritual Practice
7 Things Men Want In A Relationship
31 Reasons Why Men Don’t Cheat
Cheating Women Are Looking For Sex, Not Divorce, Study Says
Sex, Lies and Conversation; Why Is It So Hard for Men and Women to Talk to Each Other?
Each time a man connects with a woman sexually and releases his life form energy within her, he leaves a part of his information (DNA) in her birth canal.
PDF E-Book: Think and Grow Rich by Napoleon Hill