"G-D said to Moses: Say to the Kohanim, the sons of Aaron …A [Jewish] woman who is a prostitute1 or profaned2 they shall not marry, and they shall not marry a [Jewish] woman who is divorced, because each [Kohen] is sanctified to his G‑d." (Levit. 21:1,7)

...he should realize that his marrying her would expose her to untold harm...

Lubavitcher Rebbe (from a letter): In all matters of matrimony, the happiness of two partners is involved, and if there is any issue between them, the happiness of children and future generations is at stake. Obviously a marriage which is prohibited by the Creator and Master of the Universe is one that cannot be a happy one, and is certain to be harmful to all parties. In other words, if the said kohen has feelings for the divorcee in question, he should realize that his marrying her would expose her to untold harm not only in the afterlife and in a spiritual sense but also in this physical life.

The fact that it is beyond comprehension is immaterial, for it is certain that the Creator of the world knows what is best for His creatures and since He has so strictly prohibited such a marriage there can be no doubt that it is harmful. Therefore even on humanitarian grounds, the said kohen, if he has any feelings for the said divorcee [for that reason also] should give up the idea and avoid causing himself and her irreparable damage, physically and spiritually.

Copyright 2001 Chabad of California / http://www.LAchumash.org