A person who does not disturb or cause painful action in the mind of any living entity, who treats everyone just like a loving father does his children, whose heart is so pure, certainly very soon becomes favoured by the Supreme Personality of Godhead.
One who performs his duty without attachment, surrendering the results unto the Supreme Lord, is unaffected by sinful action, as the lotus leaf is untouched by water.
They (The married couples) should know that bodily relations between the husband and wife is secondary ; primary factor is that both should help one another in the matter of advancement of Krishna consciousness.
Regarding your question about the husband becoming the spiritual master of the wife, anyone who can give instruction in spiritual life is treated as spiritual master. There are two kinds of spiritual master, initiator and instructor. So the husband can help the wife as instructor.
A.C. Bhaktivedanta Swami Prabhupada
In his letter to Himavati, 24th January, 1969.