Why may one remain married if the marriage cannot bear children?
The Rebbe related the following in a letter addressed to a couple who was childless after ten years: It is written in the holy Sefarim, in particular in the Sifrei Kabalah, that although one must do all that is necessary to fulfill the Mitzvah of Peru Urevu, there are certain cases of exception in which G-d exempts one from doing so. The reason for this is because the person has a different purpose in his life [that G-d has destined for him]. The Shlah Hakadosh writes in the name of Rav Shlomo Alkabetz that Hashem sends certain souls into the world for a specific Tikkun, or leadership positions, which exempts them from having children. Not only do they not need to have children, and will not get punished for not having children, but even if they have children, the children will not spiritually be considered as theirs for any purpose. This is why Ben Azaiy did not have children, as his purpose was solely for Torah learning.________________________________________________
 Likkutei Sichos 15:480, printed in Shulchan Menachem 6:59
 See Zohar Vayeishev 188b; Shaar Hagilgulim Hakdama 3; Shlah Hakadosh in next footnote
 Shlah Hakadosh p. 381 [Parshas Ki Seitzei gloss from Rav Shlomo Alkabetz]