From the Title On the Married and the Betrothed

Chapter I. (Alexander III to the Archbishop of Salerno). After legitimate consent concerning the present, it is licit for one, even if the other resists, to choose a monastery, as certain saints called from nuptials have done, provided that carnal commixture has not intervened between them. And for the other remaining, if when warned he shall not wish to observe continence, it seems licit that he can pass to second vows. For since they had not been made one flesh, one can pass to the Lord and the other remain in the world.

Chapter VI. (The same, to the Archbishops of Strigonia and Cologne). A married man desiring to be converted in a monastery is prohibited from being received, unless his wife likewise is converted. For since after the copulation of marriage one body has been made on both sides, it is incongruous that one part be converted and the other part remain in the world.