This article is an excerpt from our Sefer
13. Every union creates new souls:
Every marital union between a husband and wife which takes place in a form of holiness creates a Jewish soul. [If, however, the intimacy is not one of a definite Mitzvah, such as one is not doing it for the Mitzvah of Onah but for pure lust, then no souls are born of this union.] This soul may enter the body of another child who is born, such as a convert, if that woman does not become pregnant with the soul that they created. It is for this reason that marital unity is permitted between a husband and wife even during times that she cannot conceive, such as during pregnancy, or after menopause, and is not considered to be Zera Levatala. Accordingly, the Zohar explains that although Sarah was physically barren, she merited to have many spiritual children.
The children of Avraham and Sarah: This is learned from the verse which states “Vatehi Sarah Akarah, Ein Lah Valad/And Sarah was barren, she had no children.” This means that although she did not have children, she gave children to others. The explanation is as follows, the souls created by the unity of Avraham and Sarah were conceived within other women’s fetuses and born carrying those souls. The male souls created entered male fetuses, and the female souls entered female fetuses. Accordingly, although Avraham and Sarah had no physical children [until the birth of Yitzchak], they had many spiritual children which were born as others’ babies on earth. These children who were born carrying the holy children souls of Avraham and Sarah eventually converted and became followers of Avraham and Sarah. This is the mystical meaning behind the converts that were made by Avraham and Sarah, as it refers to the souls that they created and were born into the bodies of those who eventually came to convert.
Children of these souls will be born in the future: Rebbe Shimon stated that a person is commanded to always be intimate with his wife [during the set Onah times], even if she cannot get pregnant, as his seed will not be going to waste. Hashem appoints an angel to guard the seed for the future, when each and every one will be entered into a body and thus the man will have many children.
The need for sanctification even when the wife cannot become pregnant:
Based on the above, it is understood that the need to sanctify oneself during intimacy applies even if one’s wife is currently pregnant or can no longer become pregnant, as nevertheless, there are souls born from each union who, in the future, will receive bodies, and these souls are effected by the sanctification of the parents. Indeed, if one does not sanctify himself during intimacy, then these souls will not be born and it ends up that his strength and power has gone to waste.
 Zohar 3:138; Rabbeinu Eliezer Ben Archa 10; Sh’lah Hakadosh Lech Lecha p. 45 and Sha’ar Ha’osyos Erech Kedusha 402; Chesed LeAvraham [Rav Avraham Azulai] Mayan 2 Nehar 65-66; Or Hachaim Vayikra 12:2; Elya Raba 240:2; Chida in Nitzutzei Oros in name of Maharam Galanti; Ben Ish Chai Vayeira 1:24; Biur Halacha 240:1 “Ela B’onah” in name of Elya Raba in name of Mekubalim; See Ma’amarei Admur Ha’emtzai Shmos 1:5; Sefer Hama’amarim 5711 p. 206; Sheyikadesh Atzmo 14:13
 Ezer Mikodesh E.H. 76:1
 Chida in Midbar Kedeimos Mareches Zayin 4 p. 83 and Chasdei Avos 6:2, based on Zohar; Teshuvas Rabbeinu Eliezer Ben Archa 10; Ben Ish Chai Vayeira 1:24
 Biur Halacha 240:1 “Ela B’onah”; Sheyikadesh Atzmo 14:13
 Implication of Sh’lah ibid; Ezer Mikodesh E.H. 76:1