I have a friend exactly in the same situation. He didn't know this girl is pregnant with another man. They got married, but his wife has been seeing this other guy secretly who got her pregnant. The guy knows its his baby. So, a couple of years past, the wife can no longer keep this a secret and told the husband directly and bluntly. He was shocked! The whole family was shocked!! They have loved the baby as their own already. The wife with head up high told the whole family. Instead of living a lie, she told the truth. She doesn't really care about the consequences as long as she is free. So, the family advice for an annulment. Fortunately, everything went smoothly.
I think its my friend's fault because he was the one who forced the marriage, otherwise he'll commit suicide. The girl, out of a pity, gave in to the pressure.