Statistically speaking, men
cheat for a single primary reason: sex.
Human beings are not monogamous by nature. They don't
mate for life with the first person of the opposite sex
they come across. And it's because of this biology, it's
believed, that many men experience the natural urge to
"spread their seed".
Now to be sure, in the case of marriages or committed
relationships, there is far more than just biology at
work. Many men report a loss of sexual desire for their
wives after children are born...or even after witnessing
their wives give birth. Their wives, to them, no longer
appear as sex objects, but rather maternal figures.
Another widely reported cause is the breakdown of fun
or spontaneity in the relationship. For example, to men
in long-term relationships sex will become routine or
mechanical. Not an adventure as it once was, but simply
as a means to an end. And so they seek that adventure,
that adrenaline rush, with another woman (or women).
Yet another cause is more biology related...the need
to conquer. Think ego. It's reported that many men will
seek sex outside of a relationship simply to see if they
can do it. To see if they can convince a woman they are
attracted to...to sleep with them. It's the "notch
in the belt" phenomenon that while frowned upon in
America, is surprisingly widely accepted in many parts
of the world.