March 6. On this date in 1955, Shoghi Effendi addressed a letter to an individual member, stating about the Bahá'í requirement to have the bride and grooms' parents' consent prior to marriage, "If you persevere and make it plain to the mother that you will not marry her daughter without her consent, it is quite possible she will become impressed by your devotion to your Faith, and give her permission. In any case, your Bahá'í duty in the matter is quite clear."

March 6. On this date in 1955, Shoghi Effendi addressed a letter to an individual member, stating about the Bahá'í requirement to have the bride and grooms' parents' consent prior to marriage, "If you persevere and make it plain to the mother that you will not marry her daughter without her consent, it is quite possible she will become impressed by your devotion to your Faith, and give her permission. In any case, your Bahá'í duty in the matter is quite clear."

If you persevere and make it plain to the mother that you will not marry her daughter without her consent, it is quite possible she will become impressed by your devotion to your Faith, and give her permission. In any case, your Bahá'í duty in the matter is quite clear.
6 March 1955 to an individual believer.

The requirement of the consent of all four parents is based on the words of Bahá'u'lláh and 'Abdu'l-Bahá...

From the Writings of Bahá'u'lláh

[65] It hath been laid down in the Bayan that marriage is dependent upon the consent of both parties. Desiring to establish love, unity and harmony amidst Our servants, We have conditioned it, once the couple's wish is known, upon the permission of their parents, lest enmity and rancour should arise amongst them. And in this We have yet other purposes. Thus hath Our commandment been ordained.
Kitab-i-Aqdas, page 42
Marriage is conditioned on the consent of both parties and their parents, whether the woman be a maiden or not.
Synopsis and Codification of the Kitab-i-Aqdas, the Most Holy Book of Bahá'u'lláh, 1st edition. (Haifa, Bahá'í World Center, 1973), page 39.

From the Writings of 'Abdu'l-Bahá

In this glorious Faith marriage arrangements which are made by parents for their children while the latter are young and have not yet attained the age of maturity are not acceptable. This matter is the responsibility of the couple when they attain the age of maturity.
From a Tablet--translated from the Persian

In the document linked, Marriage, Consent of Parents to, there are numerous other letters from Shoghi Effendi and the Universal House of Justice along these lines. Such as...

About the consent of parents for marriage: this is required before and also after the man or woman is twenty-one years of age. It is also required in the event of a second marriage, after the dissolution of the first whether through death or through divorce.
The parental consent is also a binding obligation irrespective of whether the parents are Bahá'ís or not, whether they are friendly or opposed to the Cause. In the event of the death of both parents, the consent of a guardian is not required.
10 October 1936 to an individual believer.

and

The validity of a Bahá'í marriage is dependent upon the free and full consent of all four parents. The freedom of the parents in the exercise of their right is unrestricted and unconditioned. They may refuse their consent on any ground, and they are responsible for the decision to God alone.
19 March 1938 to an individual believer.