Objection 1. It would seem that bigamy is removed by Baptism. For Jerome says in his commentary on the Epistle to Titus (1:6, "the husband of one wife") that if a man has had several wives before receiving Baptism, or one before and another after Baptism, he is not a bigamist. Therefore bigamy is removed by Baptism.
Objection 2. Further, he who does what is more, does what is less. Now Baptism removes all sin, and sin is a greater thing than irregularity. Therefore it removes irregularity.
Objection 3. Further, Baptism takes away all punishment resulting from an act. Now such is the irregularity of bigamy. Therefore, etc.
Objection 4. Further, a bigamist is irregular because he is deficient in the representation of Christ. Now by Baptism we are fully conformed to Christ. Therefore this irregularity is removed.
Objection 5. Further, the sacraments of the New Law are more efficacious than the sacraments of the Old Law. But the sacraments of the Old Law removed irregularities according to the Master's statement (Sent. iv,). Therefore Baptism also, being the most efficacious of the sacraments of the New Law, removes the irregularity consequent upon bigamy.
On the contrary, Augustine says (De Bono Conjug. xviii): "Those understand the question more correctly who maintain that a man who has married a second wife, though he was a catechumen or even a pagan at the time, cannot be ordained, because it is a question of a sacrament, not of a sin."
Further, according to the same authority (De Bono Conjug. xviii) "a woman who has been corrupted while a catechumen or a pagan cannot after Baptism be consecrated among God's virgins." Therefore in like manner one who was a bigamist before Baptism cannot be ordained.
I answer that, Baptism removes sin, but does not dissolve marriage. Wherefore since irregularity results from marriage, it cannot be removed by Baptism, as Augustine says (De Bono Conjug. xviii).
Reply to Objection 1. In this case Jerome's opinion is not followed: unless perhaps he wished to explain that he means that a dispensation should be more easily granted.
Reply to Objection 2. It does not follow that what does a greater thing, does a lesser, unless it be directed to the latter. This is not so in the case in point, because Baptism is not directed to the removal of an irregularity.
Reply to Objection 3. This must be understood of punishments consequent upon actual sin, which are, or have yet to be, inflicted: for one does not recover virginity by Baptism, nor again undivision of the flesh.
Reply to Objection 4. Baptism conforms a man to Christ as regards the virtue of the mind, but not as to the condition of the body, which is effected by virginity or division of the flesh.
Reply to Objection 5. Those irregularities were contracted through slight and temporary causes, and consequently they could be removed by those sacraments. Moreover the latter were ordained for that purpose, whereas Baptism is not.
Source: Summa Theologica (New Advent)