Religions have both mysticism (worship of a God or Gods or some entity) and a moral ethic (code of behavior).
BDSM certainly does have a code of ethics but so do Doctors, Barbers and the like. There is no "God" in BDSM or in Medicine, etc.
Therefore BDSM is not a religion.
By the way Taoism is a mysticism without a moral code and Confuciusism is a moral ethic without a God. Therefore the two are often combined. The major world's religions have both.