yevamot 11

the man shouldn't waste a good woman that someone else could claim and want to use.
and she shouldn't claim the unborn as a bridegroom.

(doing halitza, the ceremony that severs the yibbum bond and sets the widow free to marry outside of the family, is equivalent to divorce in that she cannot marry a priest. do halitza with a rival wife who couldn't marry a priest anyway is better. it is like throwing out water from one's well, when other men want to drink...)