Many people in BDSM are involved in what is known as a polyamorous relationship. Many, however, do not.
A polyamorous relationship is one in which there are more than 2 people involved. In such a relationship, all of those involved are aware of each other person who is involved and are agreeable to the relationship arrangements. It is very difficult to keep a poly relationship running smoothly, but it is possible.
I am not sure why there are so many poly relationships in BDSM, but I have my thoughts on the subject. I think that many are drawn to BDSM because of the opportunity to have more than one partner. This is not neccessarily a bad thing, but it can be if the person is not truly a dominant or submissive. Some people seem to like poly relationships because they think they are less work and require less of an emotional comittment on their part. Unfortunately, that is not true and such an attitude dooms a poly to failure.
With the number of poly relationships in BDSM, there is a misconception that the only way to live a "true" BDSM relationship is to live in a polygamous relationship. This is NOT true. There are just as many monogamous relationships in BDSM as there are poly.
One must be completely honest with themselves about their preference for poly vs. monogamy and not allow themselves to be pushed into a relationship that they know is NOT for them. Do not think you can change another's person's mind when it comes to poly or monagomy, that is very difficult to do and most efforts will meet with failure.
A BDSM relationship should be satisfying to everyone participating in it. These relationships are not cut and dried and are only what those involved make of them. There is no right or wrong way to run a BDSM relationship. So don't let someone tell you that polygamy is the only way to go, it is not. Do what you know is right for you, and you will be much happier with yourself, your relationship, and your life.
