'But officer! I tripped and landed on his dick!'
To be honest, it's a skewed religious thing (adultery being such a big crime, not marriage itself) and some people deserve to be cheated on. Of course, those who end up cheating have to pay the penalty:
Man cheats on his wife? He gives her all his money, the car and the house.
Woman cheats on her husband? She gets everything.
Oh yeah, I forgot we're still in that bullshit period where a man can never get divorced without losing all his possessions to the skank.