A cheater seeks to (hold onto what is good) in her/his primary relationship while addressing their "other needs" on the side. Clearly the upside is (getting everything you want/need). The downside is the risk of losing one's primary relationship if caught.
It's been reported that in the U.S. women file for 66% or 2/3rds of all divorces. This would indicate that an unhappily married woman is more likely to end a marriage while an (unhappily married man is more likely to to cheat). In fact the friends of mistresses often warn them; "The husband never leaves his wife."
Cheating is a cowardly act because the person is unwilling to end a relationship prior to pursuing another one. They don't want to give up their (known present) for an (unknown future). For many people it's like holding onto one job while looking for another. It takes courage to break free from a relationship without having someone "waiting in the wings."