An S and M relationship requires sickness in both parties - the sadist(Mary?) is no more the bad guy than is the masochist (Dave?). Dave is quite likely a liar and also a guy who would put words in your mouth, would tell Mary that you said something you didn't even say. I know why you said what you said and would have been tempted to do so too, but I would have been wary. I have a sister like this, and I never respond in any way when she tries to get me to talk about someone else, because I've learned the hard way that she goes right to that person and reports an exaggeration of what I have said or indicated, even if I only nodded. Stay away from them both, they are using you as a weapon in their fucked up relationship. Dave likes this shit. Oh now I'm mad you have enough to deal with. They are zombies, don't give eother of them another thought.
It's hard because I see her in the surf all the time. It's hard to avoid her. If I had my way I wouldn't see them at all .. and I'm definitely done with him too. Your advice is bang on.
I had this exact problem once upon a time. The next time I saw the wife, I went up to her with a big smile and gave her a kiss on the cheek. She started avoiding me after that.