This is true, but I believe that the majority of users on "normal" dating sites are looking for single, long-term partners. As I understand, within the BDSM scene there are several websites including social networking sites and dedicated match making sites catering to the specifics of BDSM. I find it unlikely you'd use a "normal" dating site when you likely have pretty specific interests that likely (?) need specific UI/UX to cater to.
Just sort of overall, when your interest is in building a network, finding people to have casual sex/encounters with, a "stream of people to meet" as someone mentioned below, I think you'd want a different website/UI than these big dating sites seem to offer/encourage. That said, I've never used them, just speculating based on the ads I've seen over the years and how they paint themselves.
Dating sites that make you answer questionnaires and match based on answers are a really good way to get to know people with similar kinks and interests.
While there are specific sites for BDSM dating with more nuanced optoins, the ads for generic dating sites are all very "tame" and try to not deviate from the perceived norm too much (= "find a partner, have a happy family" type messaging)
The reason is that if you do, it's virtually impossible to get included in Ad networks and App Stores.
So you naturally see only dating ads catering to the very conservative viewer.
Example: A BDSM dating site got banned from Googles Play Store after including a background image of a simple leather whip. [1]
There are couples looking for other couples or thirds, there is the BDSM scene with people looking for casual play partners, and so on.