Objection 1. It would seem that order is not an impediment to matrimony. For nothing is an impediment to a thing except its contrary. But order is not contrary to matrimony. Therefore it is not an impediment thereto.
Objection 2. Further, orders are the same with us as with the Eastern Church. But they are not an impediment to matrimony in the Eastern Church. Therefore, etc.
Objection 3. Further, matrimony signifies the union of Christ with the Church. Now this is most fittingly signified in those who are Christ's ministers, those namely who are ordained. Therefore order is not an impediment to matrimony.
Objection 4. Further, all the orders are directed to spiritual things. Now order cannot be an impediment to matrimony except by reason of its spirituality. Therefore if order is an impediment to matrimony, every order will be an impediment, and this is untrue.
Objection 5. Further, every ordained person can have ecclesiastical benefices, and can enjoy equally the privilege of clergy. If, therefore, orders are an impediment to marriage, because married persons cannot have an ecclesiastical benefice, nor enjoy the privilege of clergy, as jurists assert (cap. Joannes et seqq., De cler. conjug.), then every order ought to be an impediment. Yet this is false, as shown by the Decretal of Alexander III (De cler. conjug., cap. Si Quis): and consequently it would seem that no order is an impediment to marriage.
On the contrary, the Decretal says (De cler. conjug., cap. Si Quis): "any person whom you shall find to have taken a wife after receiving the subdiaconate or the higher orders, you shall compel to put his wife away." But this would not be so if the marriage were valid.
Further, no person who has vowed continence can contract marriage. Now some orders have a vow of continence connected with them, as appears from the text (Sent. iv, D, 37). Therefore in that case order is an impediment to matrimony.
I answer that, By a certain fittingness the very nature of holy order requires that it should be an impediment to marriage: because those who are in holy orders handle the sacred vessels and the sacraments: wherefore it is becoming that they keep their bodies clean by continence [Cf. Isaiah 52:11]. But it is owing to the Church's ordinance that it is actually an impediment to marriage. However it is not the same with the Latins as with the Greeks; since with the Greeks it is an impediment to the contracting of marriage, solely by virtue of order; whereas with the Latins it is an impediment by virtue of order, and besides by virtue of the vow of continence which is annexed to the sacred orders; for although this vow is not expressed in words, nevertheless a person is understood to have taken it by the very fact of his being ordained. Hence among the Greeks and other Eastern peoples a sacred order is an impediment to the contracting of matrimony but it does not forbid the use of marriage already contracted: for they can use marriage contracted previously, although they cannot be married again. But in the Western Church it is an impediment both to marriage and to the use of marriage, unless perhaps the husband should receive a sacred order without the knowledge or consent of his wife, because this cannot be prejudicial to her. Of the distinction between sacred and non-sacred orders now and in the early Church we have spoken above (Question 37, Article 3).
Reply to Objection 1. Although a sacred order is not contrary to matrimony as a sacrament, it has a certain incompatibility with marriage in respect of the latter's act which is an obstacle to spiritual acts.
Reply to Objection 2. The objection is based on a false statement: since order is everywhere an impediment to the contracting of marriage, although it has not everywhere a vow annexed to it.
Reply to Objection 3. Those who are in sacred orders signify Christ by more sublime actions, as appears from what has been said in the treatise on orders (37, 2,4), than those who are married. Consequently the conclusion does not follow.
Reply to Objection 4. Those who are in minor ordersare not forbidden to marry by virtue of their order; for although those orders are entrusted with certain spiritualities, they are not admitted to the immediate handling of sacred things, as those are who are in sacred orders. But according to the laws of the Western Church, the use of marriage is an impediment to the exercise of a non-sacred order, for the sake of maintaining a greater honesty in the offices of the Church. And since the holding of an ecclesiastical benefice binds a man to the exercise of his order, and since for this very reason he enjoys the privilege of clergy, it follows that in the Latin Church this privilege is forfeit to a married cleric.
This suffices for the Reply to the last Objection.
Source: Summa Theologica (New Advent)