Rules of conduct. Only certain ways of behaviour are allowed, switches tends to have a hard time both in reality as well as on forums. Also if you have one interest that is outside the norm. Animal roleplayers to mention one example are often attacked with deliberate misunderstandings even suggestions of copulating with animals by other BDSM interested individuals.
Even within one male dominant female submissive relationship you will find that the ceiling is quite low, even on a forum.
If a submissive is allowed to suggest things, then you can bet you soon will see posts telling that she 'is topping from the bottom' and that you are a crappy Master. The examples can be made manifold but I will move on to some other points.
Breaking the law, many of us in many countries do break the law with these activities. We need not look further than Great Britain to find examples of raids by the police and arrests.
This does not only account for the action of private couples at home. Some BDSM organisations do even have bylaws that go against the legislation of the society where they are found. Examples found in several countries, though I limit to only one BDSM organisation in Norway where they have some ideas I don't subscribe to, including on rape.
That we do this for one erotic purpose changes nothing, many cults do indeed have the same aim, it may be polyamory ...or things we might dont even want to contemplate here (well I spit it out,
children yuck.)
If you go against the dogma of a cult, you will be expulsed or banned.
Also that we can see here, very often there is only one view that is allowed on a number of matters.