You answered your own question within the last 15 minutes on a legal thread where a father who is not on the birth certificate, has never paid support in 7 years (I believe), has never had contact, is now engaged, he found out the child was adopted and now "they" want visitation and perhaps custody. The question of "she knew he was the father but put no name on the birth certificate, is this fraud?" has come up - can the adoption be reversed for fraud?
If you want the best for your child you'll determine paternity and THEN decide how to proceed.