The type of people who are going to cheat are going to do it anyways, regardless of any law. Those who aren't planning on it, the law won't change their actions at all. If someone doesn't respect or regard the law then there's no chance of him/her following it. In the state of Alabama (I believe) it's still illegal to have oral, anal, or non-missionary position sex. We're ALL going to jail!
and another thought...some people might claim that it's trying to "legislate morality" because in some value systems adultery wouldn't necessarily be "punishable".