Having unprotected sex with someone else when you are married. Wow! Dude what were you thinking? Life in my opinion is about cause and effect. And the effect hit you like a hammer over the head. There have been others giving their opion but I agree with some aspects of what others have said.
Here is what i do not agree with what you said. To even think about walking away when it might be yours is horrible. Which means you have considered it. To say she only has your license plate number. There are enough deadbeat dads in this country without adding another one. Do the right thing. Do get a DNA test and find out once and for all. If it is yours provide.
As ATX said you put your wife at risk here also. Damn shame.