The First Rule of Pervert Club
"The First Rule of Pervert Club is that you lose the right to judge others"
Judge not thy neighbors weird freaky kinky sex acts and M/s rituals, lest ye yourself be judged; let they who are without sin cast the first stones... or something like that.
The First Rule of Pervert Club is a pop culture reference that pays homage to the 1999 film Fight Club and indicates an attitude of inclusivity and tolerance concerning the kinks of others; ie, Your Kink Is Not My Kink and that's OK.
The phrase is most frequently used when teaching newer explorers of BDSM to be open minded about the kinks of others by contrasting their kinks to the kinks of others, showing that many vanilla folk would find their personal kinks disgusting, unsafe, abusive, insane or otherwise disruptive, and thus, if what they do is acceptable and shouldn't be condemned, then perhaps what others do in their private time and space also should not be condemned provided what takes place happens between informed and consenting adults.
While one may not understand the nuance and appeal of a particular kink, they don't have to like it in order for someone else to like it, and practicing such things is just fine for those that do enjoy whatever activity is in question.