Even supposing that is part of their appeal, and it may be to some, why is that problematic, or why does it make sexual liberation problematic? Obviously you do not think that social conventions are sacrosanct. Is the worry that if they become mainstream they will no longer be appealing?
FWIW, if you read the sex advice columns in Men's Health, Cosmo, etc. ("Ten things that turn him/her on" and the like), you will see that BDSM activities are already mainstream in the sense that they are discussed as normal and exciting things to mix into your sex life in mass market publications that are marketed to ordinary people and are not targeted at lifestyle perverts. I've never seen mummification or fisting in those pages, but bondage, spanking, leather or latex play and other less heavy activities are just part of sex, if their presence in those venues is any indication.
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