According to Ephesians chapter 5, husbands are to love their wives as their own bodies and nourish and cherish them as if they were their own flesh. How many husbands do you know who tie themselves up, whip themselves, spank themselves, etc., etc. What would you think of a man who did such things to himself? You'd think that he has serious problems, but when he does it to his wife, then it suddenly becomes okay or normal? You also said that pain can be therapeutic. God can bring chastisement or correction into our lives which can be to our benefit if we yield to the same, but a husband is never to inflict pain upon his wife for any alleged therapeutic reasons. Rather a husband is instructed to wash his wife with the water of the word. BDSM is demonic in nature and has no bearing upon the nature of Christ whatsoever.