The primary sense of male denotes the sex of an animal, as an adjective: men, boys, rams, bulls, and peacocks are all male; women, girls, ewes, cows, and peahens are all female.*
The sentence that you are asking about uses male in an extended sense. Since boxing is not an animal, you can safely infer that the author wants you to understand male not in its primary sense, but in some way related to its primary sense. You must infer the exact meaning from context.
In this case, male spectacle means an activity that men, but not women, perform, which creates a visually conspicuous show that attracts other people's attention (a "spectacle"). This usage of male is similar to its meaning in male display, referring to things like the spreading of a peacock's tail feathers. The display is not itself literally male. The display is an action distinctive to males of that species; the females don't do it.
I don't understand the word male in this context to mean male-dominated, though the meanings are somewhat ambiguous. The primary (though rare) sense of dominate is to hold power over someone; a common secondary sense is to exist in the greatest number or proportion in some area. If thousands of women took up boxing, they would still be outnumbered by male boxers, and so the sport would still be male-dominated in that secondary sense. Men might still control the sport, too, so it would still be male-dominated in the primary sense. I understand male spectacle in the quoted sentence to regard boxing as something like the peacock's tail display: an activity performed exclusively by men. However, the phrase and context are not fully clear about this, and I doubt that the author thought about it carefully.
Note that, in this context, there is a slight difference in meaning between male spectacle and masculine spectacle. Masculine denotes the qualities distinctively associated with maleness, but not maleness itself. It's not a self-contradiction to say that a certain woman has some masculine qualities—for example, a deep voice, or an urge to get into fistfights. Through the choice of the word male or masculine, the author guides the reader to understand the phrase by extending one primary sense or the other: the male sex, or the qualities associated with it. The choice of male rather than masculine agrees with the author's meaning of "Boxing was formerly performed only by men, but now women are starting to box, too."
* By the way, English has a peculiar ambiguity regarding the words animal, man, cow, and peacock. One sense of animal is any organism that moves itself from place to place and has consciousness, human or otherwise; another sense of animal means only non-human animals. One sense of man includes humans of both sexes; another sense, much more common, means only male humans. One sense of cow includes all animals of the genus Bos; another sense means only females of the genus Bos. Even the word peacock commonly refers to both sexes of the peafowl, despite the fact that -cock specifically designates a male bird. The word peacock is well-known, peahen is not well-known, and peafowl is almost unheard-of in everyday English.