Indeed a delicate question, and I hope one by which the questioner can find a loving resolution.
Here is a wiki page on the topic: http://bahai9.com/wiki/Satisfying_one%2 ... ge_partner
(and its child link, http://bahai9.com/wiki/Physical_and_spi ... n_marriage
Notice the quotes like:
"some may need to restrain this quality [of sensuality], others may need to foster a greater warmth of feeling."
"He suggests to you that perhaps you are not giving your husband enough of your love, physically and spiritually, to keep his interest centred in you."
"The true marriage of Bahá'ís is this, that husband and wife should be united both physically and spiritually".
That being said, in another connection, the House of Justice did state, "The consummation of marriage by a couple is, as you aptly state, an intimate and private matter outside the scrutiny of others." (see http://bahai9.com/wiki/Consummation_of_marriage
Bahá'u'lláh has urged marriage upon all people as the natural and rightful way of life. He has also, however, placed strong emphasis on its spiritual nature, which, while in no way precluding a normal physical life, is the most essential aspect of marriage. That two people should live their lives in love and harmony is of far greater importance than that they should be consumed with passion for each other. The one is a great rock of strength on which to lean in time of need; the other a purely temporary thing which may at any time die out."
(From a letter written on behalf of the Guardian to Mr. John Stearns, January 20, 1943--the first pioneer to Ecuador)
The above on sex being a private matter and subject to potentially dieing out, does not I think negate at all the earlier words and referenced quotes on marriage also needing to be a physical union.
I think the balance is well described by Shoghi Effendi in a letter on his behalf:
"The Institution of marriage as established by Bahá'u'lláh, while giving due importance to the physical aspect of marital union, considers it as subordinate to the moral and spiritual purposes and functions with which it has been invested by an all-wise and loving Providence. Only when these different values are given each their due importance, and only on the basis of the subordination of the physical to the moral, and the carnal to the spiritual, can such excesses and laxity in marital relations as our decadent age is so sadly witnessing be avoided, and family life be restored to its original purity, and fulfil the true function for which it has been instituted by God."
(From a letter written on behalf of Shoghi Effendi to an individual believer, May 8, 1939: Family Life, pp. 18-19)
And maybe this might apply (in connection with give and take):
"In any group, however loving the consultation, there are nevertheless points on which, from time to time, agreement cannot be reached. In a Spiritual Assembly this dilemma is resolved by a majority vote. There can, however, be no majority where only two parties are involved, as in the case of a husband and wife. There are, therefore, times when a wife should defer to her husband, and times when a husband should defer to his wife, but neither should ever unjustly dominate the other. In short, the relationship between husband and wife should be as held forth in the prayer revealed by Abdu'l-Bahá which is often read at Bahá'í weddings: 'Verily they are married in obedience to Thy command. Cause them to become the signs of harmony and unity until the end of time.[+F1 (Bahá'í Prayers, p. 107, 1982 ed.)]'"
(From a letter written on behalf of the Universal House of Justice to the National Spiritual Assembly of New Zealand, December 28, 1980)
So, in summary, although this is a private manner (unless a couple wished to seek advice about it), and it of course would most definitely not justify any slight kind of forceful attempt at resolution, from a moral point of view, willfully and perpetually withholding sex appears to me by the above to be clearly against the spirit of the Writings of the Faith.
Even a lack of attraction is presumably not an excuse for failing to seek the advised physical union with one's partner since divorce is not permitted on grounds of a lack of attraction or sexual compatibility/harmony: http://bahai9.com/wiki/Divorce#Divorce_ ... nd_wife.29
As far as sin, the Faith may not have the concept of "original sin" nor a preponderance of overwhelming and non-constructive guilt and shame sometimes associated with the word, but the concept of "sin" exists--it is used frequently in the Writings in the same sense as in the wider society: an evil action for which repentance should be sought and future actions corrected (e.g., "Certainly for an intelligent man death is better than sin", SAQ, p. 265