Digest of Biblical Civil Law

What Jesus commanded about a man not putting away his wife [Matt. 5:31-32], is not contrary to what the OT Law prescribed. The OT Law was unwilling that a man should put away his wife, since it prescribed a delay, so that excessive eagerness for divorce might cease through being weakened during the writing of the bill [Deut. 24:1]. Hence Our Lord, in order to impress the fact that a wife ought not easily to be put away, allowed no exception save in the case of fornication.

- Thomas Aquinas, Summa Theologica, Q. 107, Art. 2, http://www.newadvent.org/summa/2107.htm.