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”.