Oww! Tough one, I really don't think you can bring it up in an easy way without more or less giving away your own bdsm leanings - he'd feel suspicion or jump to it if you tried to argue your case with some force. YUnless the opportunity really falls into your lap or he says something similar again, I'd say leave it, sorry.
It does aggravate it that he's a psychologist, but some psychologist do have this heavily biased opinion of the s&m thing - Sheldon Bach for instance (a clinical professor of psychology at New York University and supervising analyst at the New York Freudian Society) asserted that S&M is an addiction where people will feel a compulsive need to be "anally abused or crawl on their knees and lick a boot or a penis or who knows what else. The problem," he continues, "is that they can't love. They are searching for love, and S & M is the only way they can try to find it because they are locked into sadomasochistic interactions they had with a parent" That's as stubbornly put as the religious convictions of a Fundamentalist Christian, and highly insulting to boot.
I know this is a hard choice, but if you bring the matter up without feeling that it comes unforced, you may both end up feeling very uncomfortable, you feeling that you've let down your guard and exposed your sex life to your dad and he just won't understand any of it. Which would leave a permanet scar, maybe a feeling too that this is always hanging between you: is she/is she not/does he...?
Just like Alex I don't think his ideas will make a big difference to his clients in the long run. In any talk relation/treatment with a psychologist it would take a long time before opening up about your s&m leanings and most people, even if they are feeling at a low ebb, will recognize blatant uninformed preconceptions and simple bashings about their lifestyle when they hear it.