The amazon fantasy is all about role reversal.
If you're a manly man, you do manly things, like hold down a job, head your family, discipline the kids, and move the couch when the wife wants to redecorate. You're expected to be strong, dominant, and independent. Everybody relies on you, and believes that Father Knows Best.
Now if the women in the audience will please stop snickering, I can continue.
If a woman is going to hold any power in a relationship with her manly man, she's going to have to do it through skillful subterfuge or withholding sex. She isn't as educated, doesn't make as much money, isn't as strong, and doesn't know how to change a flat tire.
You in the back, I said stop snickering. Reality is not to intrude upon fantasy, and you're not allowed to make fun of it, lest someone make fun of yours.
As I was saying. The amazon fantasy reverses the traditional roles of power in male/female relationships. The woman is taller, stronger, and more dominant, both physically and in personality. Don't confuse this with dominance in the sadomasochistic sense, though. The man in an amazon fantasy is typically not a sniveling submissive, eager to please his mistress. Rather, he's often a willful and proud man, reduced to a pitiful state by someone larger and more powerful than he.
The subject of an amazon fantasy is a very tall, strong woman, at least in comparison to the man involved. She usually plays the role of a bully, wrestling, trampling, and even raping men while gloating about her power and making humiliating comparisons to her victim's lesser size and strength.
There's a lot of crossover here with other power-related fetishes. An amazon fantasy may contain female bodybuilders, plumpers, torture, bondage, humiliation, foot fetish, rape fantasy, and more. Drawing deeper from the fantasy well and getting farther from reality, we can find superheroines, giantesses, transformation, and vorarephilia. On the other side of the spectrum, we can find gentle amazons. Oh, they'll still beat and humiliate their men, but afterwards she'll care for him like a baby, with all the associated rights and privileges thereof (and no more).
There's a video market for this stuff, but nothing as big as the Anal Invaders series or Lusty Lesbian Librarians, which I believe is up to part XXXVII and still with no resolution to the story's primary conflict in sight (will Natalie ever get over her sexual hang-ups, I wonder?). Women in these videos are usually very tall, close to or over six feet and ranging from hefty to BBW. The men are invariably short and scrawny types, and look even smaller when the woman is wearing those platform heels that are de rigueur in these flicks. The women could wrestle, trample, face stomp, or smother her puny victim, always in such a way as to accentuate the size discrepancy between the two. They're available from specialty distributers on various internet sites. Just don't expect the latest big-budget special effects, some of these places are lucky to be able to afford the camera.
So what in the heck is this all about?
For some men, it might be about a temporary escape from the responsibility inherent in being a man in modern society. The same stereotype I presented above means that the man has a lot of weight on his shoulders to bring home the bacon, fix problems for people who depend on him, and act as both judge and role model for any kids he might have. The idea of someone forcibly taking that responsibility away could be an escapist fantasy. He's been absolved of responsibility, but not of his own free will.
On the other hand, it's possible that some just like to see other men humiliated, and have no interested in being humiliated themselves. The character of the victim isn't an audience surrogate for them, just a guy who's getting abused.
Then again it could be deeply Freudian, involving unresolved mother issues manifesting themselves in a need for a powerful female figure to punish them.
Or are they closeted gay men, rationalizing away their desires to be ravaged by someone more muscular and dominant by thinking of their hypothetical partner as female?
Quite frankly you could sit around and come up with meaningless rationalizations all day. Fantasies don't have to be rational, or logical, or even sane, and a guy who likes to watch a video or read a story about something wouldn't necessarily want it to actually happen to him. The simple fact of the matter is that some guys like this, and therefore there's money to be made in fulfilling it. Why? It doesn't really matter.