My reasoning.

If I was the Husband 1. I would want to know if my wife is having unprotected sex with another man. 2. It would squash any lingering doubts and force me to Man up Legally. 3. I wouldn't have to have the awkward conversation where I ask for one.

If I was the Other Man 1. I want to be apart of my child's life. 2. It would squash any lingering doubts and force me to Man up Legally.

If I was the Mother 1. I want to know who I have the right to go after for child support.

If I was the child 1. I would want to know who my father is as soon as possible. 2. This knowledge might help with any future health issues I may have.

There are many more reasons why I think this should be law and I can't really think of a reason as to why it isn't law (Embarrassment, No more Murry YouTube Clips). If you have any points you would like to add to either side please comment.