January 28. On this date in 1977, the Universal House of Justice addressed a letter to an individual believer wherein they note that "Bahá'u'lláh did state that the primary purpose of marriage was the procreation of children," "A decision to have no children at all would vitiate the primary purpose of marriage," and that "Sterilization...is not permissible in Bahá'í law except in rare instances where it is necessary for a medical reason."

 

January 28. On this date in 1977, the Universal House of Justice addressed a letter to an individual believer wherein they note that "Bahá'u'lláh did state that the primary purpose of marriage was the procreation of children," "A decision to have no children at all would vitiate the primary purpose of marriage," and that "Sterilization...is not permissible in Bahá'í law except in rare instances where it is necessary for a medical reason."

1163. Husband and Wife to Decide How Many Children to Have
"There is nothing in the Sacred Writings specifically on the subjects of birth control, abortion or sterilization, but Bahá'u'lláh did state that the primary purpose of marriage was the procreation of children, and it is to this primary purpose that the beloved Guardian alludes in many of the letters which are quoted in the compilation. This does not imply that a couple are obliged to have as many children as they can; the Guardian's secretary clearly stated on his behalf, in answer to an enquiry, that it was for the husband and wife to decide how many children they would have. A decision to have no children at all would vitiate the primary purpose of marriage unless, of course, there were some medical reason why such a decision would be required.
"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 than 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."
(From a letter written on behalf of the Universal House of Justice to an individual believer, January 28, 1977)

When reading this note in Lights of Guidance, I am drawn to the notes preceding and following this one.
The preceding note states "Birth control, however, when exercised in order to deliberately prevent the procreation of any children is against the spirit of the Law of Bahá'u'lláh, which defines the primary purpose of marriage to be the rearing of children and their spiritual training in the Cause"
The following notes state

1164. Vasectomy to Avoid Having Unwanted Children Not Permitted if It Results in Permanent Sterility
"Directly to your question about having a vasectomy, in general it is not permissible to have a surgical operation for the purpose of avoiding having unwanted children if such an operation could result in permanent sterility. While circumstances might exist in which sterilization would be justified, this does not appear to be the case with you."
(From a letter of the Universal House of Justice to an individual believer, May 30, 1974)
1165. Tubal Ligation
"The Universal House of Justice has received your letter of April 29 asking about tubal ligation and has noted that you are familiar with general Bahá'í principles on the subject. However, it has directed us to say that under normal circumstances it is not permissible to have a surgical operation for the purpose of not having more children if such an operation could result in permanent sterility."
(From a letter written on behalf of the Universal House of Justice to an individual believer, May 28, 1978)