Some of the guys I know who cheated had almost a split personality. They had a personality for when they were with girlfriends and a different one when they hung with friends. Like "relationship George" in Seinfeld.
There were others who married nice girls but went to the massage parlors for happy endings (this I know about from my so). Essentially they had a nice relationship with their girlfriend/wife and other relationships for just sex. The guys I knew that cheated wanted a traditional wife to raise their children. When our group went to bars and stuff their wives/girlfriends never came. I new another guy who had a wife and a full blown relationship with a girlfriend who was pregnant. I have no idea how they juggled it.