(NSFW) What's the psychological thought behind S&M/BDSM being an alternative sexuality (as opposed to being the outgrowth of "daddy" or similiar issues)?

I’ve heard many times that S&M doesn’t mean you have a lack of mental health, it’s just what gets you off, but I’ve never heard the psychological theory behind this – I’ve only heard the theory of the opposition. Anyone have any insight into this?