Not that I am a promoter of polygamy, but I strongly disagree with you here. If a guy does not have courage to step up and marry someone openly (second marriage, that is), and opt for secretly sleeping with another woman behind his wife's back, I am not sure what kind of security, he is man enough, to provide to his first wife.
You're missing the point. Even marrying another woman openly is an open statement that his first wife is not good enough for him.
A second wife takes half of his time, a mistress may only take 3% of his time. He is taking half of his money (which was once for the first wife and her household and children), and giving it to another. He is dividing his time as a father (thus, the children see less of their father and have to rationalize why he doesn't love mommy enough that she's his only wife), and he strips them of their inheritance rights if he has children with another woman. He strips the first wife of her status, and brings her public shame.
It's theft of love and rights. Man and woman were created to be a couple. You can't be as close as clothes for eachother if you've got a whole closet to cover.
I'd rather my husband cheated on me.