There are several theories for which.
Men are simply not good at assessing a woman's level of interest in her, says Alexandra Katehakis, founder and clinical director of the Los Angeles Center for Healthy Sex.
"Yes, they are bad," she told CNN. "And (the pictures) are not welcome."
In addition, says Katehakis, most women are not interested in penis images at all.
"They are more excited by (the pictures of) a man's eyes or his buttocks," she says.
Joe Kort, a sex therapist and sex specialist in Royal Oak, Michigan, says something about it. "Most men really thought that women would be open and eager to see (a photo of the penis) and that it would make them sexually horny.Of course, this does not happen for most women."
They are attached to their private parts
The New York psychotherapist, Jeannette Stern, said that this misperception is also fueled by the attachment of men to their private parts.
"I think men, usually more than women, feel a connection to their genitals and want the person who is of sexual interest to share that interest," Stern said. "I think men perceive this as more enjoyable for the recipient than it is often."
They can get a thrill
Some men are eager to share photos of their private parts with complete strangers, says Katehakis, director of clinical services. They often do so through what is called cyberflash.
Some men will use for this the AirDrop function of the iPhone. AirDrop allows iPhone users to receive photos from nearby people if the feature is enabled and set to receive images from everyone.
It's just a hi-tech version of male flashers from years past.
"This is not so different from old school exhibitionism," said Katehakis. "We think of the terrifying guy in the trench coat – it's the modern version of that."
In some cases, this is born of hostility
Katehakis sees this as an act of rage against women, now filtered through the cybersphere.
"For the human being, it is above all about power and control, an act of sexualized hostility," she said. "Men who take their rage against women in an erotic form."
The shocked reactions of the women who get these images are a source of great emotion for the men who sent them, said Kort, the sex therapist.
It can be question of domination
Russell Stambaugh, a sex therapist in Detroit, explains that sending photos of private parties is a combination of search for acceptance and intimacy, boasting and domination seeking.
"Since bragging and seeking dominance are less conventional expressions of women's gender roles, it is not surprising that men … do it much more often," Stambaugh told CNN. "Because it's often an unsolicited search for domination, genital photos of guys are far more often considered aggressive."
For many, it's an impulsive thing
Many men suffer from a lack of impulse control, said Stern, the New York therapist.
"Impulsivity plays an important role in this regard, as is the use of drugs and / or alcohol." A more impulsive person is more likely to send the photo, especially under the influence, while a person less impulsive might want but thinks next, except perhaps under the influence, "she told CNN.
"Thrill-seeking behaviors with a partner are safe, but cybercommunications are never safe, and that seems like a lesson that's worth repeating."
There is a basis for evolution to this
Are some men just cabled to do these things? Stambaugh is almost certain, saying that there is an evolutionary basis for certain behaviors of this type. He calls this the "sexual signal" designed to stimulate the conditions from which reproductive sexual behavior can occur.
Justin J. Lehmiller, a sex educator and researcher, shares this view and explains in a blog how the so-called error management theory works.
But Lehmiller adds that men should not use this as an excuse for bad behavior.
"Even if we think that this behavior is adaptive to the meaning of evolution, it does not mean that it is acceptable or excusable that men send women photos that they do not want to see. "he wrote.
After all, Katehakis said, we to have evolved.
"It probably has biological bases," she said, "but we have developed these big brains to deny all that."
Note: This story has been updated to credit information sources.