If she doesn't want an abortion, it's her body, and hers to decide. Have the DNA test once the baby is born (yes, it's expensive in utero -- risky too); chances are it's not your baby. But if it is, you have another person to LOVE. Lucky you, make the best of it. That child could become the light of your life, you just never know.
My rule of thumb is, never have sex with someone you wouldn't have a baby with. I mean, use every precaution, but always remember it is always
My SO had "sex with the ex" 2 years after their divorce and 6 years after his vasectomy..... they ended up pregnant, remarried, and back in the same miserable state of matrimony they were in before! -- The resulting child, however, ended up being the best thing that ever happened to him.