I misread your post, as well. But, now that I think I know what you're getting at, here is my two cents...
I think it boils down to communication. Too many people in vanilla relationships have a hard time being truly honest not only with themselves but with others. There are times when some of them have awakenings and realize what they desire is more on the lines of BDSM but they fear that their partner wouldn't understand and the communication between them shuts down and the communication between the explorer and the BDSM community begins. And, the BDSM community has been shut out so much that they aren't going to shut someone else out for whatever reason especially when this person is just beginning to find their true nature. I think, simply, the BDSM community seems like it would simply be more open than the partner would to some people and in many ways they are. Why are they? Because they are much more honest with themselves (at least, most of them are) that certain things like polygamy (or multiple partners) are very natural and that "infidelity" is something created by the mainstream society and is not true to nature.
Yes, there are monogamous BDSM relationships but many are set up like nature where a male/Master is polygamous and the female/sub is monogamous. Strict monogamy was developed by the church not by nature and I think most BDSMers do have some grasp of that and are less judgemental of "infidelities" because of this than society is.