For a subculture, there are:
1) common sex acts ppl consider normal, admit to in public
2) common sex acts ppl are ashamed of, hide
3) uncommon sex acts
So 1 is "vanilla" (normal) and 3 is "kinky" (deviant). 2 is a grey area that people lie about a lot. It's normal, but the official line is it's deviant.
Someone claimed BDSM is kinky. Actually, some significant chunk of BDSM is normal in many subcultures. Some people seem to think BDSM is in category 3. But tons of it is category 2, and some is category 1.
You can see BDSM on mass market TV, for example USA's show Satisfaction.
Neil and Simon legitimize their partnership while Adriana introduces Grace to the world of BDSM.Among other things, they have like a jail cell built into a house they lock a girl in and then use BDSM toys on her, on the show. It's softcorn pornagraphy meant to arouse its mainstream audience.
And who makes this? Comcast owns the TV channel. Comcast is huge and also owns NBC. Comcast's market cap is $150,000,000,000.
And most of the viewers feel so naughty watching it. That's part of the appeal. It's on fucking mass market tv. It's normal! But they somehow feel it's naughty at the same time.
Keeping things a big deal somehow is part of how ppl keep lasting interest in it. It keeps the excitement. It helps avoid it staleness. Over-hyping the amount of deviance of an activity is one way to keep it seeming like a big deal. It makes it more special, secrative and important.
It wasn't that long ago that, legally, you couldn't rape your wife. Marital rape excemptions ended in all 50 US states in 1993, but different legal treatment of marital and non-marital continues to this day in some states. Yet ppl act like rough sex is a rare, weird preference.
It wasn't that long ago that beatings were common. Beating children. Beating slaves. Beating wives. Is it any surprise at all that these ideas aren't all gone?
It's not like these things stopped b/c ppl were thoroughly persuaded and fully rationally understood something better. They didn't become Objectivists. It got suppressed in various ways, and lots of ppl are half-persuaded. It contradicts some liberal ideas with some popularity, but people don't really understand liberalism that well or thoroughly.
The ppl who are half-persuaded beatings are bad are a great target market for beatings-sex-play. It's toying with an issue they think is important and are conflicted about!
From the female side a bunch of the appeal of BDSM is like "you wouldn't do that. no way. that's off limits. omg u did! that's so intense!"
Some of the male side of BDSM is similar to parents who say "because i said so" and maybe hit their kids now and then. It's getting sex "because i said so", without reasons.
Some of the appeal for both sides is faking reality – pretending he's so great and appealing and dominant and alpha and worthy of submission to. Like a great leader, a great head of household you can trust and follow.
But they are using whips and ropes b/c that's all false and he has trouble getting her to submit, at all, without the props. E.g. he doesn't feel confident she won't stop sex at any moment if she isn't tied up. And she doesn't feel confident he'll keep her in the mood if she isn't tied up. She may want to stop.
Like William Godwin explains, using force is a confession of weakness. That still applies when it's fake.
Let us consider the effect that coercion produces upon the mind of him against whom it is employed. It cannot begin with convincing; it is no argument. It begins with producing the sensation of pain, and the sentiment of distaste. It begins with violently alienating the mind from the truth with which we wish it to be impressed. It includes in it a tacit confession of imbecility. If he who employs coercion against me could mould me to his purposes by argument, no doubt he would. He pretends to punish me because his argument is strong; but he really punishes me because his argument is weak.Also girls like having no choice about sex. By which I mean pretending they have no choice. And the guys like feeling they have given the girl no choice. That makes them more manly and able to take or get what they want in life.
Things like ropes, beatings and rape fantasies (rape fantasies are very common, but commonly not admitted) help pretend it isn't voluntary. Putting effort into pretending it isn't voluntary is, by the way, such a fucked up thing. It really clashes with liberal values. That's one of the reasons people lie about it in public so much.