Married women cheat quite a bit and children born from these affairs are often raised by the father who never knows the difference.
Doctors run across this all the time.
The odds of the child having brown eyes from 2 blue eyed parents is lower than the odds of the woman having a sexytime partner outside of the relationship and then raising it within the marriage.
Lastly, you absolutely do not need both parents to perform a paternity test. If you want to check if you are the father all you need is your DNA and your child's. It will be extremely obvious if you are or aren't the father. (this is assuming you don't need consent from the mother).
But you certainly don't need her DNA to test if you are the father (not scientifically at least).
The final question: Do you really want to know?
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EDIT: Nevermind... posted this without reading whole thread.