The Kitáb-i-Aqdas - The Most Holy Book
Index term: purpose, under Marriage
k63.
God hath prescribed matrimony unto you. Beware that ye take not unto yourselves more wives than two. Whoso contenteth himself with a single partner from among the maidservants of God, both he and she shall live in tranquillity. And he who would take into his service a maid may do so with propriety. Such is the ordinance which, in truth and justice, hath been recorded by the Pen of Revelation. Enter into wedlock, O people, that ye may bring forth one who will make mention of Me amid My servants. This is My bidding unto you; hold fast to it as an assistance to yourselves.
n88.
God hath prescribed matrimony unto you.
Bahá'u'lláh, in one of His Tablets, states that God, in establishing this law, has made marriage "a fortress for well-being and salvation".
The Synopsis and Codification, section IV.C.1.a.-o., summarizes and synthesizes the provisions in the Kitáb-i-Aqdas and Questions and Answers concerning marriage and the conditions under which it is permitted (Q&A 3, 13, 46, 50, 84, and 92), the law of betrothal (Q&A 43), the payment of the dowry (Q&A 12, 26, 39, 47, 87, and 88), the procedures to be adopted in the event of the prolonged absence of a spouse (Q&A 4 and 27), and sundry other circumstances (Q&A 12 and 47). (See also notes 89-99.)
n133.
It is forbidden you to wed your fathers' wives.
Marriage with one's stepmother is here explicitly prohibited. This prohibition also applies to marrying one's stepfather. Where Bahá'u'lláh has expressed a law between a man and a woman it applies mutatis mutandis as between a woman and a man unless the context should make this impossible.
'Abdu'l-Bahá and Shoghi Effendi confirmed that, while stepmothers are the only category of relatives mentioned in the text, this does not mean that all other unions within a family are permissible. Bahá'u'lláh states that it devolves upon the House of Justice to legislate "concerning the legitimacy or otherwise of marrying one's relatives" (Q&A 50). 'Abdu'l-Bahá has written that the more distant the blood-relationship between the couple the better, since such marriages provide the basis for the physical well-being of humanity and are conducive to fellowship among mankind.
n134.
the subject of boys
The word translated here as "boys" has, in this context, in the Arabic original, the implication of paederasty. Shoghi Effendi has interpreted this reference as a prohibition on all homosexual relations.
The Bahá'í teachings on sexual morality centre on marriage and the family as the bedrock of the whole structure of human society and are designed to protect and strengthen that divine institution. Bahá'í law thus restricts permissible sexual intercourse to that between a man and the woman to whom he is married.
In a letter written on behalf of Shoghi Effendi it is stated:

No matter how devoted and fine the love may be between people of the same sex, to let it find expression in sexual acts is wrong. To say that it is ideal is no excuse. Immorality of every sort is really forbidden by Bahá'u'lláh, and homosexual relationships He looks upon as such, besides being against nature. To be afflicted this way is a great burden to a conscientious soul. But through the advice and help of doctors, through a strong and determined effort, and through prayer, a soul can overcome this handicap.

Bahá'u'lláh makes provision for the Universal House of Justice to determine, according to the degree of the offence, penalties for adultery and sodomy (Q&A 49).