Primary Factor For Married Couples
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.
Husband As The Instructor
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.
Guidance
Spiritual master is representative of God. Similarly, king is also representative of God. The father is also representative of God. These are the statements of the sastras. Because they will guide. The king will guide. The spiritual master will guide. The father will guide. What is that guidance? That guidance is how one can become Krishna Conscious by education, by culture.
Consulting The Spiritual Master
The Lord in human society does what is the duty of everyone, but sometimes He does something extraordinary and not to be imitated by the living being. His acts of evening prayer as stated in Srimad Bhagavatam must be followed by the living being, but it is not possible to follow His mountain-lifting or dancing with the gopis. One cannot imitate the sun, which can exhaust water even from a filthy place; the most powerful can do something which is all good, but our imitation of such acts will put us into endless difficulty. Therefore, in all actions, the experienced guide, the spiritual master, who is manifested mercy of the Lord, should always be consulted, and the path of progress will be assured.