A wife and a girlfriend are two totally different titles. Wanting a paternity test for a girlfriend is pretty valid, especially if you had previous relationship issues. Actually, if you had marital issues [cheating, separation etc.] then I would advise for a paternity test.
Example: I know this 25? something old girl who has two children. She told the guy she was dating since high school that both kids are his and he signed his name on the birth certificate stating such. The children are 5 and 3? All these years, through the break up and everything, he has paid hundreds of dollars a month towards child support for his children, even though he's not with their mother any more. That doesn't happen often, so props to him. Though, the mom is keeping a dirty little secret. He's not the father of the oldest child but she won't tell him that. That is the grimy.
If you've never had relationship problems, then there's no reason to not trust your wife.

shadha-
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You may shoot me with your words,
You may cut me with your eyes,
You may kill me with your hatefulness,
But still, like air, I'll rise.
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