From a 1974 essay Sontag wrote about Leni Riefenstahl, Fascinating Fascism:

In pornographic literature, films, and gadgetry throughout the world, especially in the United States, England, France, Japan, Scandinavia, Holland, and Germany, the SS has become a referent of sexual adventurism. Much of the imagery of far-out sex has been placed under the sign of Nazism. Boots, leather, chains, Iron Crosses on gleaming torsos, swastikas, along with meat hooks and heavy motorcycles, have become the secret and most lucrative paraphernalia of eroticism. In the sex shops, the baths, the leather bars, the brothels, people are dragging out their gear. But why? Why has Nazi Germany, which was a sexually repressive society, become erotic? How could a regime which persecuted homosexuals become a gay turn-on?

A clue lies in the predilections of the fascist leaders themselves for sexual metaphors. Like Nietzsche and Wagner, Hitler regarded leadership as sexual mastery of the "feminine" masses, as rape. (The expression of the crowds in Triumph of the Will is one of ecstasy; the leader makes the crowd come.) Left-wing movements have tended to be unisex, and asexual in their imagery. Right-wing movements, however puritanical and repressive the realities they usher in, have an erotic surface. Certainly Nazism is "sexier" than communism (which is not to the Nazis' credit, but rather shows something of the nature and limits of the sexual imagination).

Of course, most people who are turned on by SS uniforms are not signifying approval of what the Nazis did, if indeed they have more than the sketchiest idea of what that might be. Nevertheless, there are powerful and growing currents of sexual feeling, those that generally go by the name of sadomasochism, which make playing at Nazism seem erotic. These sadomasochistic fantasies and practices are to be found among heterosexuals as well as homosexuals, although it is among male homosexuals that the eroticizing of Nazism is most visible. S-m, not swinging, is the big sexual secret of the last few years.

...

The rituals of domination and enslavement being more and more practiced, the art that is more and more devoted to rendering their themes, are perhaps only a logical extension of an affluent society's tendency to turn every part of people's lives into a taste, a choice; to invite them to regard their very lives as a (life) style. In all societies up to now, sex has mostly been an activity (something to do, without thinking about it). But once sex becomes a taste, it is perhaps already on its way to becoming a self-conscious form of theater, which is what sadomasochism is about: a form of gratification that is both violent and indirect, very mental.

Sadomasochism has always been the furthest reach of the sexual experience: when sex becomes most purely sexual, that is, severed from personhood, from relationships, from love. It should not be surprising that it has become attached to Nazi symbolism in recent years. Never before was the relation of masters and slaves so consciously aestheticized. Sade had to make up his theater of punishment and delight from scratch, improvising the decor and costumes and blasphemous rites. Now there is a master scenario available to everyone. The color is black, the material is leather, the seduction is beauty, the justification is honesty, the aim is ecstasy, the fantasy is death.

Interesting that she had noticed the connection even before many radfems. Sheila Jeffreys, for instance, said she "became aware of the links between sadomasochism and fascism in 1981" and wrote the paper, Sadomasochism: The Erotic Cult of Fascism, in 1984, in which she argues:

The erotic roots of fascism lie in the way in which sexuality under male supremacy is structured in individuals. Because western male supremacy encourages us to experience sexuality as an immensely powerful and nigh uncontrollable force, the erotic aspect of fascism has great significance. We do not learn to express ourselves sexually in a world of equal, loving relationships. Women and men are born into the heterosexual system of male dominance and female submission.