For the time being it is, and it’s likely to remain that way. (Although women’s brains are sensitive to overstimulation, too.) Men have some characteristics that render them especially vulnerable to the seduction of Internet pornography.
In 2002 Canadian researchers compared brain images of male and female subjects while they viewed erotic film excerpts. The males reported significantly higher levels of physical arousal. While both genders showed activation of similar corresponding brain regions, only the men showed significant activation of the hypothalamus.
[This research suggests] that the greater sexual arousal generally experienced by men when viewing erotica may be related to the functional gender difference found here with respect to the hypothalamus.
The hypothalamus is part of the reward circuitry of the brain, but its function is much broader. It is the headquarters for integrating the body and the mind. It’s the command center for the endocrine system and the autonomic nervous system. It is the seat of desires, emotions. It’s a player in every thought, emotion, desire and impulse. It controls all of the sex hormones.
Men are vulnerable
In short, men are especially vulnerable to porn images because these images can hijack a male brain’s command center in a way they generally cannot a female’s. The hypothalamus is also the center that determines hunger and satiation—in both sexes. 
So, an arousal response to Internet porn is as natural to a man as a growling stomach is to the smell of food on a grill.
Now that it’s clearer how men are wired, consider the implications of today’s flood of porn images. The human brain has never had to contend with such an assault, which is strategically calculated to stimulate this vulnerable part of the male brain. A few short decades ago young men usually got their first glimpses of bare breasts in National Geographic. Then came “Playboy,” “Hustler,” X-rated films, hard-core porn, and now the free, never-ending novelty of the Internet (not to mention virtual-reality porn). In effect, today’s porn viewers are guinea pigs in a mass experiment. It is quite possible that male brains are not well suited to handle this overload of erotica without losing their equilibrium.
More proof of this innate evolved mechanism in the male brain comes from our monkey cousins. See Male monkeys will “pay” to view female monkey bottoms
Role of Testosterone
Testosterone is powerful stuff, and men typically have ten to twenty times as much of this hormone as women. Said one female-to-male transsexual who raised her testosterone to male levels in connection with a sex change,
I felt like I had to have sex once a day or I would die. … I was into porn as a girl, but now I’m really into porn.
Porn images are likely to raise testosterone, but domination themes certainly do—perhaps because male brains reward men for striving for the alpha male position in a tribe, troop, or other group. Whatever the reason, the result is that domination themes in porn are as calculated as lacing cigarettes with extra nicotine; they make porn more addictive to men.
Deliberate manipulation?
Some suggest that porn makers deliberately use imagery that raises testosterone levels in the viewer. Testosterone tends to make one more lustful (testosterone raises dopamine, the craving neurochemical), more irascible, and less fully in control. For a fascinating first-hand account of its effects, see this article. A 2015 meta study found that porn use is associated with increased aggression, and a 2016 study found that those with compulsive sexual behavior are more aggressive than other addicts. Does porn use condition sexual tastes to aggressive acts, or does it appeal to more aggressive users, or both?
It appears that men have greater vulnerability to addictions that are highly stimulating. In a 2006 study, men released markedly more dopamine than women in response to amphetamine. This helps explain why stimulating activities such as watching sports, off-track betting and violent porn hook men so easily. It seems that evolution favored selection of the genes that encourage men to pursue and conquer things.
For all these reasons, men are particularly vulnerable when it comes to fast-paced, hyper-erotic, hardcore Internet porn. Yes, porn makers are also learning how to appeal to female viewers, but women may always be less vulnerable to porn addiction, in general.
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