As to the possibility of conception without the presence of a male sperm in the future; this is a question which lies entirely within the province of science, and which future scientists will have to investigate.
(Shoghi Effendi, Lights of Guidance, p. 288)
As to your desire and that of your husband to avoid any action which would permanently prevent you from bearing children, the only text we have so far found on the subjects is in a letter to an individual believer from the beloved Guardian. The question asked was whether after a few children it would be permissible to have a surgical operation on the wife to prevent further conception. His reply was that such an act was inacceptable and unworthy, and those who commit the act were responsible before God.
(Universal House of Justice, Lights of Guidance, p. 346)
It should be pointed out, however, that the Teachings state that the soul appears at conception, and that therefore it would be improper to use such a method, the effect of which would be to produce an abortion after conception has taken place.
(Universal House of Justice, Lights of Guidance, p. 344)
O SON OF MAN!
I loved thy creation, hence I created thee.
(Bahá’u’lláh, The Arabic Hidden Words 4)
Regarding your question concerning the passage in ‘Seven Valleys’ referring to pre-existence. This in no way presupposes the existence of the individual soul before conception. The term has not been absolutely accurately translated, and what is meant is that man’s soul is the repository of the ancient, Divine mysteries of God.
(Shoghi Effendi, Lights of Guidance, p. 505)
Veiled in My immemorial being and in the ancient eternity of My essence, I knew My love for thee; therefore I created thee, have engraved on thee Mine image and revealed to thee My beauty.
(Bahá’u’lláh, The Arabic Hidden Words 3)
You and your husband, therefore, should have no feeling that you are obliged to add to your already large family. This is a matter entirely for you to decide, and there are many methods of preventing conception, including self-discipline and restraint, to which you can have recourse. Sterilization, however, would be a more far-reaching action that any of these, with implications and results beyond those necessary for the immediate purpose of limiting the size of your family, and is not permissible in Bahá’í law except in rare instances where it is necessary for a medical reason.
(Universal House of Justice, Lights of Guidance, p. 347)
... the Bahá’í Writings affirm that the human soul comes into being at the time of conception. However, they do not clearly define the exact biological moment and nature of the event described as conception and this may, indeed, be a question that is insoluble by human thought or investigation, since it relates to mysteries of the spiritual world and the nature of the soul itself.
(Universal House of Justice, Reproduction and other Biological Subjects, 6 February 1997, to a National Spiritual Assembly)