Why didn't the gemara answer that the case where the relationship of the husband preceeded the relations with the adulterer was a case when the husband performed the kedushin with an act of "BEAH" ?
Well done! Your question was asked by the Minchas Chinuch (Mitzvah #365).
He answers that the Bi'ah must be a Bi'as Ba'al, in other words, they would have had to be married before the Bi'ah took place. Therefore, the Bi'ah of Kidushin does not qualify. The OHR SAME'ACH (Ishus 10:1) also discusses this questions, and tries to prove that from this Gemara that Bi'ah Nesu'in Oseh (see Kidushin 10a).