No, it's not right of you to judge people in that way. So someone does something unusual in their free time. As long as everyone involved has given informed consent, there is nothing wrong with it.

Whatever blows their skirt up, as Sammie would say.

As for what propels them, well what propels anything unusual like that? I don't know whether this is a sexual thing or not, but there are plenty of nonsensical paraphilias around the place. Lots of people do lots of strange things; it's often difficult or impossible to know WHY one enjoys something, only that one does.